I personally voted for 3: White Kyurem, the Therian forme of Thundurus and Keldeo's Resolute forme. I was actually really tempted to go for Landorus' Therian forme too, but that'd be a bit over-kill. XD Anyway, I love White Kyurem a lot, and just like the contrast between fire and ice. It's also my favourite out of the too and why I'll probably choose Pokemon White 2 (iirc they can both be received in both games, but to me the cover legendary does play a large part in what game I buy. :x) Also, wow, its stats are pretty impressive too. :O
Thundurus' new forme simply (in my opinion) looks much better than its original forme and I could actually see myself using it. :D Finally, Keldeo's new forme looks pretty cool too- although it's only the aesthetics that were changed. XD