One choice only. The single choice for me and I haven't really thought about it now is Prince Naseem. His highlight reel is as good as any.
If its just one choice, for all time... I suppose Id have to go with Pacquiao. Factor in his actual performances in his prime were almost always exciting, dynamic, and fan friendly.... always in shape, always pressing and pushing the fight., and with the power to have edge of your seat moments at any time. A rare combination of an elite fighter with aggression, ambition, pride and an exciting style. Then on top of that the sheer number of fun, exciting or meaningful fights he had against a whose who of champions that were in one way or another meaningful, exciting, or fun. Mab 1 Morales 1 Morales 2 Morales 3 Hatton Cotto Margarito Thurman JMM 1 JMM 4 Were all, imo, pretty fun and spectacular events, for different reasons. While not my all time favorite fighter (top 5 tho) its hard for me to think of another fighter that had as many impactful fights that left the night buzzing. So for sheer combined enjoyment throughout his career, id go Pac.