Hmm. Well I'd have to say that Rachel Summers has always been one of, if not my favorite characters. I'm not really sure why. I guess part of it would probably be because when I first discovered her I was a teenager and firmly affixed into my Angst frame and as far as I was concerned she had the most angst-ridden past of any character I'd ever seen. Then when I got older and got tired of angst, I looked at her character again and saw the evolution shewent through over the years, from the lonely, scared, angry, timid war survivor in huge baggy clothes to the happier, more confident young lady in the tight leather minidresses, to the happy, centered, strong woman in jeans and cute shirts that she is presently. That was a person who had more right to be angry and bitter and self-destructive than anyone and instead of keeping her that way, they allowed her to let go of it and move on. I just thought that was really neat, but I'm a goober that way.
As for characters I dislike, I'm not sure. I never really got into any of the hugely buff guys with really really big guns that popped up everywhere in the 90s. Except Deadpool, but he was funny and not quite as overbearingly muscled. It's been so long since I read any X-comics that I'm not really sure if I can answer that question, no one's really popping out at me. What I do hate, while it's not a character, is the drawn out love triangle between Scott, Jean, and Logan. That needed to die a lonnnng time ago. And now, what, Scott's shacking up with Emma Frost? Good lord, people. OOO! And what was the deal with M and Penance from Generation X? M I never really cared that much for but Penance was always pretty cool and then it turns out that Penance was really M and M was really their younger twin sisters masquerading as M? What?? Marvel really needs to adopt DC's system of retconning every once in a while, there are way too many dead people with evil twins running around in this universe. Sheesh.
Pardon my little rant, there.