As far as any member of the Fantastic Four go, I think an exception should be made and they should all by one single entry.
Sure, The Thing could stand out as he is the poster boy for "cursed by their powers" characters.
But the F4 are such a tight unit (tee-hee) and their debut into comics had such a big impact, I think they should be together.
Sure maybe you can say the same for the X-Men, but their impact wasn't till later. Plus, they switch out members so often the "X-Men" is mostly a concept. Same could go for most superhero teams.
But the F4, despite the occasion roster changes, are seen as the original four member family more than anything else.
Then again it ain't my list so do whatever you want.
