To the other good mentioned qualifications, I'll add
- ability to adjust and change plans depending on what happens in a fight;
- keeps a decently busy schedule, without multiple year+ stretches of inactivity;
- has great competition and overcomes it;
- no ducking tough opponents;
- not only beats other elite fighters, but beats lower-tier fighters convincingly;
- a loss or two is ok but they get avenged.
It would be nice to add, capable of winning multiple ways. However, a lot of really great fighters didn't actually do that, even if they were very well-rounded. So, I can't disqualify GSP because (mostly) all he did was control and outpoint people. Or in boxing, Mayweather, Ali, etc. - they were well rounded enough to beat all kinds of opponents, but almost always by control, as opposed to turning out their lights.
Anderson won multiple ways, he had it all (except maybe really great competition) but the PED crap blots it all out like a giant asterisk.