The Last Mission is a short movie sort of thing that bridged the gap between FFX and FFX-2, and was added to FFX International, which is only available in Japan--if I have my facts straight, anyway. PlayStation Magazine released a demo disk shortly before FFX-2 came out that included Last Mission on it; I'm sure there's also places you could download it online, if you really want to see it. Having seen it, I really don't think it's anything special. It's more like a teaser for FFX-2 than anything else.

Anyway, to answer the question, I'd have to go with FFX-2. I actually liked FFX-2 quite a lot, despite the whole Charlie's Angels-esque corniness of it. It didn't take itself seriously, and was completely different from the rest of the series, but yet it was actually fun to play. (As long as you're not concerned with getting the elusive 100%, that is. Bah.) FFX, on the other hand, managed to be the only FF out of the ones I've played (VI through X-2) that I actually didn't like.