Tyson was the most exciting to watch for brutal KOs. For two-way action fights, you've got guys like Morales, Pacquiao, Holyfield, Vazquez, Margarito, Gatti, Ward, etc... Mosley's generally an entertaining fighter, but I wouldn't rank him all that highly on the "most exciting fighters" list.
I'm amazed that nobody has mentioned Michael Katsidis. He may not be at the very elite level but this guy always gives his absolute all. You won't see a more exciting fight than when he fought Graham Earl.
surprised only one person mentioned Gatti. I guess it depends which fighting style some people prefer. Tito and Gatti both were known to get off the canvas and had amazing come from behind victories, some of them being down right brutal. RJJ was more a perfect display of athleticism that couldn't be matched and seemed unhuman alot of times, that's why I'd pick him. Erik Morales also had some pretty unforgettable decapitations like the one he did to Angel Chacon. Shane I only hope is getting a second breath of fresh air. Maybe the negative stuff that is surrounding his life now might of revived his boxing career as well as him being trained by Naz Richardson. Some would say Margo was taylor made for him and that might of been true, and even though he has more difficulty with boxers or movers, he is still dominating like he was with Collazo, an awkward southpaw technician who just lost a controversial decision to Berto. Currently, I have Sugar Shane and the Pacman as well as Dinamita Marquez as the most exciting fighters who have always delivered at the highest levels of the game, not just destroying mediocre competition.