I think that there are a lot of characters out there that are really cool and interesting, but for some reason, they don't get used all that often. They're more niche characters, but a lot more can be done with them, and it's always cool when they show up. Here are some on my list:
#Exodus- I like the character, but sometimes it seems that he can be written as a real maniac (Kind of like Silver Age Magneto?). He's very powerful, and he's not a full-out villain (Like The Shadow King or Sinister.). I think that Mike Carey did some really good work with him.
#Selene- One of my favorite villains. I'm not even sure why, I just like her. Maybe it's because of the whole magic angle. She used to be a pretty big enemy back in the day, but sadly she's not used that much anymore.
#Black Tom Cassidy- I like his friendship with The Juggernaut
#Meggan and Widget- I guess that you can say the same for some of Captain Britain, but these two just seemed to have gone to limbo and haven't come back.
#Living Monolith- Whatever happened to him? This guy can be quite a threat if he's written well. Even more-so than The Juggernaut.
#Chamber- Whatever happened to him?
Those are all that I can think of right now. Does anyone else have any underused or underrated characters that they would like to see used more often?
Lucifer - The alien who crippled Professor X. He was supposedly killed behind the scenes by Dominus in West Coast Avengers,but he can easily be brought back if someone really wanted to revive him. I suggested to the Squirrel Girl crew about bringing Lucifer back by having him possess one of SG's squirrels,but they didn't use it. If the X-Books don't want to use him then it's ok,he can cause trouble for the other do-gooders of the MU.