X is by far my favorite Final Fantasy game. Such a great story, characters, and gameplay. Easily worth the price by itself.

X-2 is not as good but consider it a kind of post game extra and it's a nice bonus addition. Play if you want more but X is perfect standalone.

Last Mission is really boring IMO but it's literally such a insignificant extra that it's not even in the title. :P Don't bother playing it.