Ooh, that's a tough one. I have quite a few favorite series, such as Pokemon, Dept. Heaven (where I got my username from), Touhou, Tales of Symphonia/Phantasia, etc. However, out of all of them I believe I only had one temporary obsession during the time I've been obsessed with FT (yes I keep track of these things OK). That would have to be Final Fantasy IX. Personally, if it weren't for Dissidia I probably wouldn't have considered downloading it onto my PSP. But anyways, I love IX for its characters, its gameplay compared to other FF games (i.e. certain equipped items will give you certain skills and stuff and you can permanently keep them after mastering them even if the item is removed, freaking Trance mode and the special abilities that come with it...), the music (OH THE MUSIC; it's really beautiful and well-done for a PS1 game and honestly I could hear it being played in an anime)... Yeah. Ironically I still haven't beaten it but I'm lazy OK.
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Chaval, where did your username come from?