I would never personally use Self-Destruct. It's just not worth it to be a man down for the rest of the fight. Ending a battle with it could make sense, though.
I happen to like Mighty Guard and White Wind. Those, and their variations, have always been my favorite Blue Magic-style abilities. It's so helpful to have support/buffs coming from a character that is usually physically oriented.
Stone Breath and Bad Breath are also fun. Meh, I'm bad at picking favorites.
Stone Breath, definitely. Random fiends and demi-bosses, too. Invaluable against the big bad at Lake Macalania.
BTW, if you don't know this tactic, search for "wendigo" in my FFX Shortlist. http://www.esque.com/slr/gamefaqs/ffx_shortlist.html
Mostly I stick to the attacks targeting multiple foes. Bad Breath is a purely vindictive pleasure for true annoyances, or to hit bosses with everything and find out what sticks (on a par with inverting remedies in FFXII). --- Guides & other contributions: http://www.esque.com/slr/gamefaqs/ FFX, FFXII, KH, KH Re:CoM, KH2, Okage: Shadow King