I bought X-2 first, and while it was a fairly decent game I still think X shites all over it in terms of storyline and gameplay. I haven't played either of them in a while, but I;d probably be more likely to play X from the start, as the plot more than makes up for how linear the first half of the game is. I also found Tidus' fish-out-of-water buffoonery pretty amusing, though the less said about the forced laughter when you leave Luca the better, I still fucking cringe any time I hear that.
I also seem to be one of the few folk on here who actually quite enjoys playing XII, due to the more open combat style and license system being an interesting change.
If folk want to talk about properly linear though, then XIII wins that hands down. This is one instance where I actually prefer the sequel.