Gun to my head, I'd pick Xillia. Xillia 2 is coming to the States soon as well, so it will have a continued story, unlike the other two.
Graces has Lineage and Legacies which wasn't in the original, which is kinda sorta its sequel.
I've never played Ni no Kuni, but Xillia was disappointing. It's a decent game, but no better than average. It's also probably the shortest mothership title in the series. Graces has some cheesy dialogue, but an awesome battle system and loveable characters.
Finished all 3, NnK is by far the best. Note that either Tales aren't better than Vesperia. Actually NnK and Vesperia are my fav two HD JRPGs. BTW, both Xilia & Graces don't have an overworld like Vesperia but NnK has.