I pretty much hate reality shows, but one that I do find reasonably enjoyable is Shark Tank. The conversation (whether from the people who have a business or between the "Sharks") is intelligent, "normal", conversation that I can either relate to, or that I can learn a little something from. The people on the show have a sense of humor and come across like adults who can disagree and still like each other. I suppose... The Extreme Make-Over shows aren't all that horrible. I don't watch them, but at least the idea behind them involves trying to help people.
The worst reality shows are the ones with a bunch of people acting like children, misfits, empty-headed competitors over meaningless foolishness, and/or emotional misfits. Equally bad: A bunch of people competing for a boyfriend/girlfriend and acting as if the the whole scenario is "romantic". Also bad: Shows that exploit what "losers" (in one way or another) people are.
Not quite a horrible as those of the worst variety (above) may be the ones like Undercover Boss or Secret Millionaire; but I'm not a fan of "making fools of" people for television purposes, and then having someone look like "a big, benevolent, cheese" by giving the people money. The undercover boss and/or secret millionaire could do the very same thing, do the right thing, and not make a big show out of it. So, to me, making a show of it pretty much cheapens what goes on with those particular shows.