...honestly, I like the concept of Mystery Dungeon the best, but I only played the first one and I'm not sure if the later games used the same ideas. I didn't get that impression from reading about them, though.
And I like the concept of Conquest, but found it frustrating because the game wants me to use Pokemon I don't really like, and also because I find it difficult to get the characters I want.
So I'd probably go with Snap, because it was the first Pokemon game I ever played.
Official Bride and Wife of Noire (of the Fire Emblem Awakening message board)