Obtaining a full Pokédex tends to be less about how many types of Pokémon are available in each game (and for the record, X/Y now seem to hold the record for most natively available Pokémon), and more about the legendaries. Particularly now that in generation 6, the GTS will allow you to search for Pokémon that you have not yet seen. Also, keep in mind that there are varying degrees of "full" when it comes to the Pokédex, because there are 11 'known' event only Pokémon held hostage by the whims of Gamefreak/Nintendo (Gen6 adds 2-3 more to this count).
Before embarking on the adventure, it is recommended to restart a couple of times in order to transfer two of the starters to Generation 4, leaving the third and final one to be transferred with the legendaries and various other Pokémon after you complete the adventure. Yes, this means you should have unlocked the Pal Park in a gen 4 game prior to starting. Otherwise be prepared to transfer the legendaries and then restart to get the starters. But getting the starters out of the way first means that should you discover your Pokédex missing some non-legendary gen3 Pokémon, zipping around Hoenn to grab them with the game already completed is a breeze.
Emerald is practically required for the generation 3 legendaries, as you have access to both Groudon and Kyogre (although Heartgold/Soulsilver blunt this requirement a bit). Pick up the three Regi's, and when your mom asks for confirmation on which she saw (Latias or Latios), your answer should depend on which generation 2 remake you have; choose to have Latios roam Hoenn if you plan to get/have HeartGold, Latias if you're getting/have SoulSilver.
On the face of it, Platinum is probably the superior choice given that it has both Dialga and Palkia in one game. However, to my knowledge the Wii downloadable title "My Pokémon Ranch" has never received the platinum update outside of Japan, so there goes one chance at obtaining an otherwise event-only Mew and semi-legendary Phione. If you're lucky though, In Diamond/Pearl they do show you the other version's legendary, meaning you could theoretically look it up on the GTS. Finding a non-stupid trade offer this late in the game's life cycle is problematic, however.
Otherwise, after you've transferred things from Emerald, Regigigas becomes available, and regardless of which of the three games you have; Azelf, Mesprit, Uxie, Girantina, Heatran, and Cresselia are all available.
Additionally if you have the side game Pokemon Ranger (Fiore/Almia/Oblivia) completing it can net you a Manaphy egg by connecting to any of the Gen4 games. Manaphy can bred with a Ditto for a Phione egg.
If you didn't start this process owning one of these games, and you've already done Emerald, which of these two you get may depend on which of the Latias/Latios pair you have. (Again, if you have Latias get SoulSilver, If you have Latios get HeartGold) In an ideal world you'd have two DS systems available, start both your D/P/Pt and HG/SS games at the same time and reset one of the games a few times transferring two starters to one game, get the final starter, transfer all the starters to that game and reset the second game a bunch of times, netting you all 6 generation 2 and 4 starters. Transferring up to Generation 5 and re-starting is also a viable option.
As far as Legendaries go; the second generation legendaries Ho-Oh, Lugia, Suicune, Entei, and Raikou are all available at various points in both games. First generation legendaries Articuno, Zapdos, Moltres, and Mewtwo should all be available after beating the Elite Four and exploring the Kanto region.
I have little to no experience with B2/W2, but from what I've seen, some legendaries are considerably easier to obtain; particularly if you have the 3DS app Pokemon Dream Radar. In any case Reshiram and Zekrom are the game exclusive legendaries that will require trading (or use of the GTS) to obtain the other. Other legendaries available for this generation; Kyurem, Cobalion, Terrakion, Virizion, Tornadus, Thundurus, and Landorus
You should only need one of these four games, with Emerald, Generation 4, and the generation 2 remakes, and perhaps a little use of the GTS, you should be able to complete the entire national Pokédex as of Generation 5 with the likely exception of most of the event only distribution legendaries (Mew, Celebi, Jirachi, Deoxys, Darkrai, Shaymin, Arceus, Keldeo, Meloetta, Genesect). Which Prepares us for...
X and Y
As mentioned at the begining of this answer, the improved GTS simplifies things greatly. Any pokemon that exists withing X&Y can easily be found on it - and improved filters mean you can show only 'non-special' Pokémon requests, and you can even filter the results by requests that you can actually fill. While this also means you can search for legendaries you need to fill your Pokédex, good luck with that one. While some people do post reasonable requests, those are largely a rarity compared to the plethora of impossible to fill ones. Like a lvl 1-10 Charizard.
Another option for assisting your Pokedex filling is Wonder Trading. While you have no say in what you get, sometimes you can get rather rare stuff like Larvesta or Togepi - people often dump their extra pokemon from breeding into Wonder Trade. As an added bonus, it's a great way to collect ID numbers for the daily lottery.
Xerneas and Yveltal are the two game exclusive legendary Pokémon this time around, with Zygarde being shared. Also available late game are the one of the Kanto region legendary birds (which one depends on your starter) and Mewtwo (Although they could have been obtained in the Gen2 remakes). After transferring from generation 5 with the Pokemon Bank 3DS app, your pokedex should be complete. Also there are reports of likely suspects for distribution-only Hoopa, Volcanion, and Diancie.
Omega Ruby and Alpha Sapphire
The release of the Gen 3 remakes causes the lion's share of the above to be largely obsolete. All that is required now is one of X/Y and one of OR/AS. There has since been another question; Can I complete my pokedex only with 6th gen games?, to which my researched answer was yes. This 'yes' does have the stipulation that there are still version-locked Pokemon for both sets of games, so unless you buy all four, some online trading through the GTS may be unnavoidable. However, between X/Y and OR/AS, all starter Pokemon, all regular Pokemon, and all (non-mythic-)legendary Pokemon that count towards the completion of your Pokedex can be caught. Even the previously event-only Deoxys is now natively available in these two games.