Ali-Frazier is equally exciting, but those two were not at the peak of their powers. I go for Bowe-Holyfield 1; Round 10 is my favourite round of all time.
Fighters don't necessarily have to be in their peaks to produce great fights. Ali and Frazier may not have been the best heavyweights at that time, but they were always evenly matched and it made for some classics. Like what has been said previously about the two, Ali and Frazier did not contest fights, they waged war. Thrilla in Manila, for me
Frazier-Ali '75 is more special. They fought like only two people that hate eachother can, driven on through heat, weariness and pain only to inflict the same on the other. This fight is pride personified. I will not yield...to you. Not to you, not now, not ever. It was something to behold, and should never be done again. Holyfield-Bowe was a great heavyweight fight. On the uppermost tier of what is possible within the confines of the sport, but 'Nilla transcends that. A psychology student could major on that fight.
The Thrilla In Manilla was fought in some weird, alternate universe. There aren't many fights that flat out crush Holyfield-Bowe for excitement, but it is one. Other-worldly.
I prefer Bowe-Holyfield Ali-Frazier 3 is a fierce fight, but not much of a contest in the last couple of rounds when Frazier's fighting blind.