This is a really tough one to answer, some of my favorite fighters were champs but not all of them were all that dominant or exciting while they had the belt so I'm not sure how to answer this. For example I'm a fan of Conor but certainly wouldn't call him one of my favorite champs seeing as he never defended. Anyways with that said here's my list.
125: DJ
135: Cruz
145: Holloway
155: Nobody in particular really
170: Lawler
185: Silva
205: Jones
265: Werdum
The fact I chose Werdum for 265 means I probably should have picked Machida for 205 but w.e, Werdum is the only champ at Heavyweight that I was a fan of.