Hmmmmm.....in X/Y, I pick Xerneas since it's the sole Fairy-type legendary (until Diancie can be officially caught), it's powerful too. You could overrun both Yveltal and Zygarde fairly easily.
Need wait for the three to come out (Hoopa, Diancie and Volcanion), then maybe I could decide better...:)
P.S: Outside Kalos, I give 99% to Victini!