I always thought WW was the most entertaining to watch. Like RRxtar said, they have the speed of the small guys but still the KO power of the bigger guys.
I also find featherweight entertaining since I'm the same size as the fighters, I often imagine myself in the cage
even tho I'd get knocked the fuck out
bigger guys - kinda slow, but hard hits that make you go like "OOHH"
smaller guys - more energetic; all-out scrapping makes it more exciting