This is a much more interesting thread idea than the hundreds of "which pornstars/celebs would you like to fuck?" threads.
Here are my picks and my reasons:
Actress: I would fuck Jennifer Garner, simply because I can't stand Ben Affleck and would love to be able to brag about nailing his wife.
Musician: Probably Katy Perry. I don't like her music but she's hot, famous, and has a nice rack. If I could time travel back to the 1980s I'd nail Lita Ford, who was the ultimate sexy rocker chick at the time.
Sports Woman: Gina Carano.
TV Personality: ESPN's Erin Andrews, no question.
Model: Probably Kate Upton. If I were able to say "You know that insanely hot young blonde from those 'staring contest' commercials? Yeah, I fucked her." my life would be complete. Brooklyn Decker would be another good choice.
Porn Star: If I had a time machine I'd go back to the 1990s so that I could fuck Jenna Jameson when she was young, hot, and the most famous porn star on the planet. But if I had to pick a modern porn star I'd probably go with Jenna Haze because she's one of the most famous porn stars in the business right now, the fact that she's also hot and fucks like a wild animal are also pluses.