Ok i'm bringing this one back from the grave because it's become an issue lately. Surely there's been some fighters that, while their total careers haven't been stellar, have had good runs or nights where they were on a level that they'd never see again or be able to maintain. Greatness is defined by longevity and a bunch of other factors but there are some fighter's who didn't have traditional "primes". There's some fighters that had one night where their skills peaked and that was it.

My 5 "Greatest" Heavyweights:
Ali
Foreman (not as many title fights as some but was a factor in the best era of heavyweights and a later era)
Holmes
Lewis
Louis

My 5 "Best" Heavyweights:
Marciano- I realize this will be frowned upon but you gotta understand that despite the man having 49 consecutive victories, only 6 of them involved a title. While he might not have the resume that ali and others have, he'd be a handful for any heavyweight on any given night.
Tyson- Great power, chin, and skills but didn't maintain it long enough to be considered a great imo.
Vitali- Big, incredible chin, and unbelievable power. While i don't consider him a great because i question his heart and longevity, on the same token i wouldn't wanna make him angry 
Tua (vs. ruiz)- Again great power and would be a handful but didn't sustain his success to be considered great.
Ike ibeuchi- Another guy that would be a handful but didn't execute on the potential he had to be great.