While I love Blitz Ace, my most useful is probably Auron's tornado. None of the randomness that can plague the Overdrives of the others (Slice and Dice, Lulu's Furies, and most of Rikku's mixes can all focus on one enemy and totally ignore the others, and Wakka's are only awesome if you don't screw up the slots), it's easy to get the added bonus for, and it is an instant killer. Set his Overdrive mode to Slayer, and with his Triple Overdrive weapon he can Overdrive over and over and over, as long as there are at least 3 enemies in the enemy party. 3 deaths with Triple Overdrive and Slayer = new Overdrive use.