There was some pretty interesting, and some pretty hard boss battles in this game. I liked the Guard Scorpion primarily because of the hilarious error in what Barret advises you, and I also loved the Hundred Gunner/Heli Gunner battle because it's taken place as you descend down an elevator; pretty cool.
If you class the Midgar Zolom as a boss, then I really enjoyed the strategy in facing him early. Lost Number was pretty tough, as was Demon Gate.
But enough about me. What were some of your favorite boss battles?