Stephen Maturin
Peter Parker.
I have lots of answers to this:
- Sherlock Holmes
- Magneto (having his powers would be way cool)
- Francisco d’Anconia, Dagny Taggart, or Hank Rearden
- A couple of my own original characters
- Spock (the TOS version)
- Willy Wonka
Yeah, most of them are guys. I just happen to identify more with male fictional characters.
Glad to see I’m not the only POB fan on the thread. But I think I’d rather be Killick, and get to nag Maturin. Harpies ain’t in it, for all love.
Lots of answers, but I’ll go with Elizabeth Swann for the nonce.
So many opportunities unused, to corrupt the innocent and fluster the corrupt, muahahaha…
The man in the brown macintosh, naturally.
President Jed Bartlett
Ward Cleaver.
I would send Wally and The Beaver off to boarding school, so I could boink June silly, especially since she always seemed to be the perfect wife.
um… why?
Wouldn’t that be a little hard on the Beaver? 
Jessica from Dune, or one of the other Reverend mothers. I would seriously love to learn all those crazy things they do. and it would be cool to experience things as a female.
I was going to say Dagny. I’d be happy with any of her men (especially Francisco).
Riker aboard the Enterprise D or E. He seemed like the one that would have some fun and it would be neat to be on the ship for a week.
Right now? I’d use it to jump into one of my characters, so I could finish this damn chapter. Hop into her head for the pivotal week, see what happens, write it down.
Or does the device only work for fiction that’s already written? 
Mine would have to be Grace Hannadarko from Saving Grace.
To be able to kick bad guy butt with the protection of a cool angel like Earl on my side would be the best of more than one world.
(and it wouldn’t hurt to have Holly Hunter’s body)
Doris Thatcher from Hot Fuzz, after the conclusion of the film. Not only would my nerdy self gain valuable insight into the Nicholas/Danny relationship, their growth as police officers after the showdown, and the current status of Sandford, Doris seems to have a lot of fun when she’s not on the job: “I’ve been round the station a few times,” “I’ve had my top off in this lay-by,” “That’s me after a couple of pints,” “Nothing like a bit of girl on girl!”
Then, when I got back, I would totally defeat my boyfriend in the competition of nerdiest Edgar/Simon/Nick fan in the area. He went to the Hot Fuzz world premiere, but me, I’d have been to Sandford! I can’t bring back anything physical, can I? Because a uniform shirt or just the badge or badge-applique would be a great birthday present for the bf.
Lucille Bluth. I could take out all of my pent-up stress by emotionally manipulating half of my children into tears before my first morning martini. Then it’s drinks and cheerful backhanded compliments with Liza Minelli until it’s time to go home! Plus her clothes are fab.
On the other hand, I’d go for Lord Peter Wimsey - before he met Harriet Vane and went all angsty 
I could fancy being a rich man about town in the 20s. Umm, I wonder if a certain Viennese opera singer is around?
Question: do we, in any way shape or form, get to see the results on our leapee’s lives after we get back to our own universe? Like if I leap into, say, Batman, if I pick up a Batman comic in three months, will I see Batman’s behavior suddenly change to what I forced him to do while I leapt*?
I’m guessing “no,” but I had to ask.
Anyway, I’d be mostly tempted to screw with people. Make them completely discredit themselves, or violate everything they believe in. Like making Batman go out and murder some prostitutes, or have the Joker get himself killed…pulling orphans out of a burning building. Something like that. 
*All things considered with the state of comic plots in recent months, this would probably be well in the range of current insanity. But I digress.
I’ve got it down to three.
The 4th Doctor in Genesis of the Daleks (or any other adventure).
Daken (son of Wolverine)
or
Molecule Man (post Secret Wars).