I've only ever seen two -- the one with Kim Kardashian and the one with Paris Hilton. Of the two, the most watchable to me personally was the Kim Kardashian video. Paris Hilton just seemed like she wasn't even there, like the guy was fucking a doll or something. o_O
I don't particularly enjoy celebrity sex tapes. For some reason they always seem to be made worse than 'normal people's' sex tapes, so for me to say 'what's my favourite celebrity sex tape' is a bit like saying 'what's my favourite terminal illness'. Mind you, I do like the Imogen Thomas one.