Honestly the best hero for a beginner (I've played in small money tournaments and taught several friends from completely clueless to easily carrying public unorganized games) is and always will be Dragon Knight.
Dragon Knight is a ranged hero during fights. Melee heroes are less forgiving because they have to waste tons of time switching targets and have much more down time if they are don't know what they are doing/indecisive. DK can pretty much always be in range of anyone and hit them.
Dragon Knight has easy to use spells. He has one of the best stuns in the game. It is on an absurdly short timer, undodgeable (unlike many other spells) and has an OK range. His breath of fire is a point and click damage spell. His ultimate you turn it on when you are fighting heroes or attacking towers.
He is forgiving. In team fights often times he will be ignored until other people are focus fired down first because he is relatively hard to kill as is and his standard build (BKB into Cuirass although I'd throw in a Halberd before AC against auto attack heavy teams and finish off with Satanic/Daedalus) involves him building hybrid defense/offense items. Beginners need to learn and it's hard for them to learn if they die instantly all the time because they are in the wrong place at the wrong time like with most of the cast. DK gets to live longer.
3a. He has 0 cast time and 0 cast delay on his spells. Beginners often get punished heavily for not taking cast time and cast delay into effect and often have no idea what the two are. Cast time is how long your guy takes to wave his hands in the air or whatever before the spell is done. This can be interupted either by you canceling the command or being stunned/silenced. Cast delay is after the spell is complete there might be a bit of a delay before anything happens.
- Perhaps most importantly, because of his design, he is almost never useless under any condition or at any stage of the game. Many of the harder heroes in the game have very specific timing windows during which they must 'make something happen' or they become useless.
Ex: Clock becomes much less useful when his main targets acquire BKB
QoP and Pudge must create advantages during the window where the base damage on their spells is enough to seriously threaten everyone
Chen must push towers during the window where neutral minion puppets are not simply cannon fodder for heroes (past lv 10ish) and ideally where they are often times stronger than heroes (lv 1 - 5)
or 'stall the game out/have their team stall until they become useful
Ex: AntiMage's team must stall for him to acquire battlefury (ideally 12 - 14 mins) and treads plus manta (ideally 25 - 30 mins).
Medusa's/Phantom Lancer's team must stall the game for as long as humanly possible.
DK has the unique position of being possibly the hardest semi carry in the game because he is ranged (ranged is always better than melee), has an AoE attack (absurd combined with ranged, especially with spells like vacuum or Clock's cogs), and high agi/str end game stats. He has a stun that can be used to temporarily drop enemy DPS to 0 (this is called anti-carry) and has Halberd as a core item (also anti carry ish). And at lv 6 he can start to chip away at towers and is almost as useful as a Puck or QoP or Pudge with runes (Note that DK is played solo mid in competitive, he uses the DD and haste runes exceptionally well) although he is more comparable to a Shadow Shaman in that he pressures towers when his ultimate is up and has comparable spells (tail is like hex with worse range, breath of fire is like forked lightning but more reliable also DK's spells are much more mana efficient). However unlike any of the listed heroes who want one or two items and then end the game around LV 16, DK is fine having the game drag along until LV25 with 6 items although he is out carried by the hardest carries (although none of those heroes are useful at 6).
Because of this, beginners can be relatively active and play with relatively little pressure on a macro scale as well as the micro scale and be useful all game long.
This is one of my closest friends. He has been playing for 8 years and DK remains one of his favorite heroes. Note his 100% win rate. I highly recommend you check out his replays for pointers. He is probably easily at least in the top 5% of all players.