24. Dan Renzi, The Real World: Miami (1996)
For this list, it's important to not just include an individual’s actions during the show; his or her post-Real World career also matters. So yeah, Dan was kind of a douche on the show (the screaming match he had with Melissa was epic), but his full bitchiness didn’t come out until after the season ended and he started doing all of those shitty Real World/Road Rules Challenge spinoffs and then later started blogging about, well...himself.
What really put him over the edge, though, was the incident in 2004 where he reveled his inner Pee-Wee Herman in a public movie theater in Kansas. A true narcissist if we've ever seen one.