I'm 60+ hours in and level 27 and just made it to Rathir. I'm guessing I've seen most of the monsters already except for Balor and I haven't seen the zombies (Sons of Laz) yet.
One of my favorite parts of this game is how cool the monsters look and act.
Some of my favorites are:
Wolves - I usually don't get too excited about wolves, but these wolves have great attacks in Reckoning. Where many monsters are way too easy, wolves with their relatively low hit points always put a hurting on me when in groups of 4 or more. I love the way they rush and jump at you almost knocking you down. It's pretty challenging to beat them without bitten quite a few times and wolves + other monsters fighting together can be tough.
Jottun - Love how they look and attack. Some funny attacks too like bouncing you off their huge belly. I wouldn't mind seeing Jottun as a playable race in the MMO.
Croduk - Ugly, freaky looking thing with unusual poison attacks. The sounds of the attacks are even scary.
Lanashe - Very cool looking with the long dark hair and floating around. I wish they attacked and defended better though.
Brownies - Nasty looking little buggers. Easy to dispose of, but they sure look mean. I wouldn't mind seeing them beefed up with some magic capabilities in the MMO.
Most disappointing for me:
Banshean and Murghan - They look cool enough but way to easy and slow. I thought those Murghan were supposed to have a lightning attack. For me they just walk slowly towards you with tridents pointing out. If you stand right in front of them you they might attack - not very exciting combat.