I've been wondering which 5v5 team composition would be the best. The heroes can all be classified under various roles like AoE, Globals, Tanks, Heals, etc.
What combinations of these roles lend to more successful strategies? What roles should be focus points when building a strong and effective team for 5v5 games?
EDIT: This is really out-of-date, and I'm working on re-writing it. I don't recommend following the advice given here just yet.
In most cases, putting all your eggs in one basket by choosing champions that all fit the same role as tanks, healers, dps, or some other mechanic will not result in a good team. A good team has champions that serve different roles and have lots of synergy with their teammates.
The most important role is a tank. Without a tank it's difficult to initiate team fights and you team will lack someone to prevent speedy gankers like Master Yi and Evelynn from killing weaker teammates. Champions that fit this role have to be able to soak a lot of damage and provide crowd control in order to protect teammates. The most useful crowd control for a tank is a taunt, which will pull your opponents attention away from a teammate they are trying to take down. For this role you want champions like Rammus and Shen, although there are many other viable tanks.
Support is also very important. Support is a rather vague word but it usually means healing and crowd control. This lets support step in to help the tank keep other teammates alive, as well as serving dps when the support isn't needed. Janna is widely played support due to her AoE heal, her shield, and her two crowd control tornadoes.
Beyond those two roles your champions will be focused on dps. However, there are still some things you should balance.
Magic Damage and Physical Damage: If you don't balance the two then your opponents will stack resistance for whatever damage your team deals. An example of this is a physical damage team being entirely useless against Rammus, who gets up to 75% physical resistance and deals damage back each time you attack him. Choosing a mage like Vlad and a fighter like Ashe will be enough to balance your damage.
Crowd Control and Nuking: Crowd control keeps your targets from running, and nuking does high damage. Choosing only one or the other means you will see targets escaping easily or not taking enough damage. An example of a champion who balances the two is Morgana with Dark Binding, which nukes and roots.
Damage Focus and Damage Resistance: Champions that focus on damage tend to lack in resistance and health. If all of your dps champions are squishy, they will be targeted in team fights and likely die, leaving your team with no damage output. Balancing damage and resistance tends to be in choice of items more than champions, but choosing some tanky dps like Sion and Jax will help. Also, tanky dps can step in as a temporary tank if your team tank happens to die.
Picking champions with AoE and global abilities is not necessary. These spells are scaled to be roughly as powerful as single target spells and thus provide marginally more value in team fights, and no more value in situations where there is only one target (i.e., you are ganking or getting ganked). AoE is only super advantageous if your team is having difficulty focusing one target.
I haven't mentioned much about synergy but it mostly comes from your team's capabilities. Do you have a nuke to follow up a nicely placed stun? Can your tank be helped when he gets too deep into a group of enemies? Think of different situations and whether you are capable of coming out on top of them as a whole or only a small part. And if you want an idea of team fighting synergy (this largely promotes AoE) watch the circle of death.
This is what I consider a good team, based on my points above:
The Roles:
The only champion that doesn't entirely follow what I recommended is Galio. However, Galio was chosen because he can deal lots of dps with enough AP focus, but he also protects teammates with Bulwark and has a Taunt to help him sub in as a tank. This means Galio can be another tank, dps, or support with enough focus. His versatility makes him an easy choice for me.
One weakpoint is that the team lacks healing, but Galio, Nunu, and Morgana have self heals (Morgana has spell vamp) and Jax usually takes some vampiric item later game. Rammus is a tank so choosing a health regen item for him like Force of Nature or Warmog's Armor can be enough.
The Synergy
Other than the roles they fit these champions also have a lot of synergy, mainly in their ultimates. Rammus can initiate a large team fight, which is when Galio and Morgana step in. Morgana casts her ultimate, and Galio taunts to keep enemies in. Then Nunu steps in to cast his ultimate, and Rammus and Jax use their cc to keep opponents close to Nunu. The sheer dps is enough to completely kill or scatter a team, and despite exhausting most of your ultimates your team still has a lot of power as each champion has cc and dps.
a fighter like Ashe -- lolwut? She's a full AD carry. Also, that's kind of a weird team build -- 2 mages, 1 anti-mage AP character, and 2 hybrids. y u no ad carry? ;P
My LoL skills are not really on par with my SC2, but here is my take:
You're going to want 1-2 Tanks. You need someone to be able to take that heavy damage in Team battles, and protect your squishies, and that means a Tank (let's say you pick someone like Shen, Mundo Nunu for this). On top of that you're going to need someone to initiate. Its often nice if this person is a Tank, but as long as they're beefy enough, it's alright (let's say you pick someone like Sion or Malphite for this).
Next we're going to need some support. Usually you want a support that synergizes well with your team. If you're loaded with Spell Damage then Soraka is a good choice. If you need some extra Bang, Lux is always a goody. Personally I think you can almost never go wrong with Zilean.
Now we're down to the fun part, your Nukers and DPS. I often think that one of these should be a jungler (like Warwick or Twitch), because that brings the threat of ganking, and forces them to spend a lot of money on wards and staying in their lane (instead of ganking you). Ideally, your DPS are going to be different damage types (one AP and one AD) to prevent them from easily building a tank against you, so if you grabbed Warwick, you'll want someone like Malzahar, Ryze or Veigar.
Additional considerations
So you have the basics of the roles, but its extremely important you synergize; and no I don't mean just come up with a nice combo. Look down your list, and say to yourself: Do I have enough Stun? Do I have enough Slow? If they field an AP Yi Carry, can I lock him out long enough to kill him?
It's extremely easy to know all the roles and still not have enough of what you need. There was one time where we ran 5 Tanks against an opposing team which only had AD carries and no slows, and we all got Thornmail and when one person got low, they ran away while the other 4 covered. I think the game ended 34-2. So think through your weaknesses.
I'd say a better team set up, as opposed to Sadly Not's would be something like this: Basically - Top: Solo, Mid: Solo, Bottom: Duo and always a jungler.
Of course you can counter pick in ranked. An enemy with a Twitch and an Ashe means that a counter picked Jax could handle mid really well, because one of the two will probably mid.
You can also pick strong laning combos that can cause utter devastation, such as Sion + Taric for example.
Getting complementary champions is always a good idea. Jarvan + Fiddlesticks can wreak havoc, a Morgana to spell shield Katarina when she uses ult etc. Focusing on AoE champs can work too, although one crow from a clever Fiddle and it could be counter productive ;)
Overall I would try to have:
Tank
Bruiser
AP Nuker
AD Carry
Support
The layout could be something like the following: (the brackets are just champion suggestions, there will be more than the ones I have listed in each category)
Top lane:
Mid lane:
Bottom lane:
Jungle:
These posts are old so, here's a more up to date summary.
AD+support bot:
Strongest support currently are soraka and sona, but there are nich duo comps where you can use support like kayle and nunu if you want. the point of this is to get your ad carry as much creep kills as possible (or at least more than the other AD carry), and keeping them healed so they can stay in lane longer.
AD+utility bot:
Strongest bot utilities right now in my opinion are ap sion and ryze and blitzcrank, but i'm sure anything with a stun or hard cc would work here. the point of this combination for bottom lane is to get your ad carry kills so that he can snowball into late game earlier than he normally should.
This is where your ap carry goes, you have an ap carry so that the opposing team will forced to buy both armor and magic resist. Strongest ap mids right now are kassadin, leblanc, casseopia, karthas, galio, mordikaiser, kennen, morgana.
Mid is an important role, its goal is to get farmed while at the same time have the mobility and utility to gank top and bot if needed. If you play mid I suggest you ward both river bushes because you will usually get ganked from the jungle more than bot and top.
This is where things get interesting. a good jungler can carry even the worst players, a bad one can force a loss for some of the best. as jungler your role is to provide buffs for your carries (blue to ap, red to ad, you can take red early game) as well as gank and cover lanes that are doing poorly.
Strong junglers currently include shaco (BROKEN), lee sin, skarner, master yi, maoki. honestly anyone can jungle. just pick what your team needs, a lot of people like to jungle tanks like amumu and maoki and use their cc to get easy kills in lanes.
this is the farm war lane. pick a character with sustain and get creep kills. strong tops now include vlad (underplayed), swain, amumu, gangplank, nasus (will get countered at higher elo), dr mundo, and singed (op). As top lane your duty is simply to farm, don't leave lane before 20 minutes except to back. Your goals should be at least 100 cs by 20 minutes.
Taking all of these roles into account, you pick champions that will work well together after the laning phase (late game) as well as counter your opponents picks (in draft). this is how team compositions are drafted.
Good synergy begins with at least one tank, AD carry, and AP carry is all you need. Mix up crowd control with damage. Use wards and always get a good initiation. There's significantly more to team synergy than just this, however, which is covered in other answers and is probably too long for a singular answer.
team comp is dependent on style
there are a few common comps
strait counter picking intended to win lanes and thus the game.
Team fight comps generally around aoe.
Poke comps.
Split push comps.
Mobility comps.
Burst comps.
Initiation / dive / assassin comps
burst comps seem to be the best in seasons 3 and 4.
There are generally 3 strategies for comps: split push(mostly want to force the enemy team to split up to cover objectives), seige (want to force objectives), or roaming team fight (want to catch people between lanes and objectives)
A very common team comp in solo que that generally works well is your adc your apc and 3 players that build strait tank with slows or some thing to case the enemy to find them selfs out of position. The team who's tanks last longer win, (Or the team who loses their dps first loses)
