I've long been a proponent of making the X-Men truly inhuman. Regardless of their mutation, X-Men should be creepy and a little unsettling to be around, to really reinforce the metaphor that there is something "all-different" about such beings. And when it is something tragically unsettling, even better!
He's a little low on the list, perhaps because he is not truly inhuman looking aside from his massive disfigurement. In fact, his body is a "chamber" of psionic energy, made obvious when his face and chest ripped away to reveal the glowing innards inside. Hey, I said this list would be tragic and unsettling, right?
He's definitely inhuman, and he kind of takes the role filled by the Fantastic Four's Thing, when the Thing was always wondering if he was a monster or not. However, Rocky here isn't too hung up on his appearance by contrast. Still, since Rockslide can't really be anything BUT Rockslide, his tragic form grants him a place on this list.
Okay, so he's not really a mutant. He's an alien dragon sidekick. But let's face it, if you saw a purple iguana in some pet store flying around and breathing fire, you'd be pretty freaked out.
7. (tie) Mercury / Pixie
One is basically that liquid Terminator and the other is basically, well, a fairy. Either way, neither should have pupils when drawn correctly, and that is pretty much a lock for 7. What's interesting to note is that there aren't that many girls on this list, anyway. Usually, any inhuman mutants that are women just happen to have a different skin color (gold seemed to be in fad during Lifeguard and Stacy X's tenures.) Mercury might fall into that latter, although she is permantly transformed and therefore not truly "human," you could say.
Remember how freaky those aliens were in that movie, Signs? Well, here's an X-Man hero that basically looks just like that. Of course, perhaps what's even more unsettling is that no one seems to know how to color him consistently from book to book.
Lizard-person? Check. Bald, but only sorta? Check-plus. Grossly mutated right arm? Check and mate.
Yes, that's right. I put Maggott on this list. Maybe it's just a guilty pleasure, but I kinda like the guy. Or at least, I like the concept and the visual. The backstory became too convoluted too quickly, forcing the readers' sympathies in a kind of "very special episode" way, which killed any chance the guy had to establish himself. Still, if this list is all about inhuman, unsettling, and downright uncanny X-Men, he's gotta top the charts. His freakin' guts are embodied by giant worms, people!
The most inhuman of the original 5 founding members, Hank McCoy has never truly been "normal," even in those early days. Just a bit too blocky, a bit too stunted, with hands and feet disproportionate. Later, add blue fur, fangs, and etc. Later still, throw some cat-features on. This guy is the poster child for inhuman mutants. During his time as an Avenger, however, what was interesting was that he never experienced such anti-mutant hysteria-- in fact, he was regarded as a bit of a celebrity. When he IS played for inhuman ferocity, however, no X-Men can really compare.
2. Glob Herman
Man, I love this guy. And he showed up in, what? Less than a dozen issues? And always pretty much in the background? There was always something so unspokenly tragic about him, and therefore both unsettling and comical at the same time. He was brought into the X-Men about the time that all mutants were acknowledged as being something more than just pretty-people-with-powers, and as such his very presence forces that issue. (Although you can say this about pretty much all the young mutants introduced by author Grant Morrison, Glob Herman gets bonus points for straddling that bizarre/loveable line.)
and 1: Nightcrawler
I doubt you could really find a member of the X-Men that best embodied the spirit of what the X-Men really were all about. This guy was at times an outcast and a friend, a hero and a monster, a leader and a faithful supporter. His gimmick was that he looked, moved, and yes even smelled, like a demon, making him the most unsettling of the core X-Men. And yet, he was also the most stalwart and purehearted, making him the true example of a hero at the same time. Congrats, Kurt Wagner!
Whew! This list making takes too long. All those other sites make it look so easy!