Well, Eric and Kelso were both taken away, and that hurt the show a lot. So obviously either one of them.
Fez added on to the show in many ways, and kept a lot of the story lines going. He also added in a lot of humor that helped make the show what it was. So not him.
Hyde can't be taken off, because without him, there's no circle (or last season of the show). And if there's no circle (or last season), there's no That 70's Show. He can't get taken off.
Donna isn't as good looking as Jackie by any means, but she moves the story along more than anybody except for Eric. This means taking her out of the show would ruin the show before it even started. There's actually an entire episode after her and Eric break up for the first time that shows that.
Jackie's personally my favorite character, but means the least to the story. There are a lot of things she does that move the show along, but really it could be somebody like Pam Macy who does that. However, her good lucks and stuck up richness makes her funny. Which is what the show's supposed to be. But the show does need a story, so...
If you're going to make the story better, than get rid of Jackie. But if you're going for comedy and good looks, then get rid of Donna.