Boy, thats tuff, I like a lot of battle systems. I really like the ATB system. Normal turn based battles are boring for me, but if they add some variety, turn based can be a lot of fun. FF X's system features a creative turn based system, and I love it. Some of the battles can be very intense, although my favorite part is the character switching.
Final Fantasy Tactics has a great system. I haven't ever played it, but my friend has it and has shown me some battles, they look like a lot of fun.
Xenogears has an awesome system, I love how the combo's work and everything. I wish I owned it...soon, maybe. I actually plotted out a sequel to the game (doesn't follow plot line, just system). It started out as just another plan for an RPG until my friend convinced me to base it off of Xenogears. I wish I had a way to submit it to them, see if the like it...get a job...
Summoner has a fun battle system. Real time and you control one character while the others auto fight, and the chain attacks are great. Although the best part of that game is when the characters parry. They sword (or other weapon) actually blocks the attack, its really quite impressive to watch.
Thats all I can think of right now, though I know I'm missing something.