I assume you mean when they were in their prime so I'd say:
Fedor
CroCop
Wanderlei
Chuck
Rampage
Lee Muray
Giant Silva 
phydeau - Matt HughesI'm about to make ur fucking day bro. Once again you shall get to watch rich franklin and Tito Ortiz for they have not yet retired my friend!
Rich Franklin
Tito
Chuck
Frank Shamrock
Tank Abbot
Mo Smith
Don Frye
Dan Severn
Ken Flo. 
Carwin.
Liddell.
Daley.
Swick.
genshi - Randleman
.
All mentioned here I guess but...
Chuck
Saku
Coleman
Kerr
Igor
Fedor
Brock
Frank Shamrock
Frye
Bas
Mo Smith
Goodridge
Hughes
Probably a lot more I'm forgetting.
Serra and Randleman straight up
Chuck for sure and Chris Lytel
Forrest griffin, fuck the haters
Igor
Goodridge
Frye
Sak
Ken
Frank
Chuck
Igor
Genki
Saku
Chuck
pre-matt serra gsp.
I miss some of the guys that fought before mma became more like a sport. There's a lot more athletes in today's mma. Back then, the dudes that we watched fight were 100% fighters. Don't get me wrong, those old guys were athletes too, but it was different back then.
Guys like....
Saku
Don Frye
Telligman
Ensone
TK
Mo Smith
Pulver
Tanner
ect
ect
ect
And I REALLY miss ground and pound with head butts...Coleman woulda finally killed someone if they didn't take out head butts.
Crazyhorse Bennett
... But seriously... Fedor by far
Vovchanchyn!
Brick-thrower
Jens
Carlos Newton
B.J.
Lee Murray
Charles Crazy Train
And Most of all Charles Mask Lewis! ( Loved that dude)
Oh ya and Jeremy Horn!
vandelay - Arturo Gatti.Fuck right. That guy was never in a boring fight.