Coleman has nothing to prove fight wise. He is only trying to prove to himself that being about 42 he can improve and be a top fighter again. It is a mental game for him. I like him but he is not a top 5 HW.
He has already:
Been the open weight UFC tourney winner 2 times.
Been UFC heavyweight Champion 1 time.
Won the first ever Pride GP.
He really has nothing to prove. At various times he could successfully argue he was the best fighter on the planet. He is battling with his mortality and I hope he adjusts his thinking and gets into other areas of the sport gracefully, rather than going into a Dan Severn mentallity of fighting smaller and smaller shows until you are in your 60's.

I think that's sad.
This is not fully up to date, but still good info:
http://en.wikipedia.org/wiki/List_of_UFC_champions