To me it goes: WW > MW > LW > LHW > HW.
I think the WW and MW divisions have guys who are still quick and agile, but retain enough size and power to pull off vicious subs or nasty KO's. It's actually odd since 2 of my top 3 favorite fighters are in the LW division (Gomi and Penn), but overall WW and MW are my favorite.
Fights like the Clay Guida/Tyson Griffon fight kill me sometimes on the LW division. I'm sitting there screaming at the tv, "Somebody cause some freaking damage! You're like a bunch of hyper hamsters!" There are a handful of LW's who can really dish out the pain, but overall I think the division is a little light on powerful guys. There are some, like Joe "Daddy," Sherk, etc., but I like the divisions where it's a norm and not a rareity.