I thought Hammerlock was more difficult, but it wasn't really as much fun as the other two.
Torgue was freakin hilarious. Which isn't to say Hammerlock wasn't funny too (IMO it WAS funny), but I dunno... maybe it's because I used to watch pro wrestling and kept expecting Torgue to tell me to snap into a slim jim.
Scarlet's was also good as well, I don't mean to take away from that either.
I saw all three as pretty blase with Torgue the most boring. Torgue is great if you like large open areas with not much going on and constant shouting in both volume and in text with a great deal of profanity. Torgue does have a good boss to farm however in Peete and really good for co-oping to keep boredom reduced. Just my thoughts.
Hammerlock is about the same but at least the enemies are more unigue and a bit deadlier.
Torgue does give up some good juicy tidbits on Moxxi if you pay attention.