A complete fighter is one who can both avoid punishment and dish it out. While pound-for-pound king Floyd Mayweather hasn't displayed fight-ending power very often at welterweight, his right cross still has more than enough snap to help him completely control the flow of most of his fights.
A complete fighter can dissect an opponent like a surgeon and set up his big punches over the course of several rounds. But he can also dig in and exchange furiously in the pocket, if that's what the situation forces him to do.
This isn't a pound-for-pound top-10 list, but it is pretty close, and everybody on it could easily be ranked that high within the next year. Like a five-tool player in baseball, a complete fighter is an all-star by definition.