I would definitely start with Black and White 2. You can find the same amount of Unova Pokemon in White 2 as you can find in White 1 I believe, plus you get a free Zorua, and there are many Pokemon from every other region that can be found. Like ZoroarkMaster132 said, the GTS is a great way to get other Pokemon that you can't find, just watch out for the abundance of offers that need level 9 legendaries and the such. It can be very reliable though. Using a 3DS to get Thunderus, Tornadus, and Landorus in their Therian formes via Dream Radar also helps out as well. There really isn't much more to add I guess.
If you're playing White 2, having a copy of White 1 to transfer Zekrom and the other starters is also something you can do. The same is for Black 2 and Black 1.