From all my experience, my top 10 most exciting I've seen are..
1.Nigel Benn
2.Ricky Hatton
3.Manny Pacquiao
4.Mike Tyson
5.Tony Ayala (That dude was sick!)
6.Johnny Tapia
7.Felix Trindad
8.Miguel Cotto
9.Jorge Arce
10.JLC/Chico for one of the all time great fights. You can't say one more than the other!

I've probably missed a few out, but that lot all together are balls out, iron chinned, KO merchants who come from the brink, and stick it all on the line in virtually every fight.