Personally I've always been partial to Mona and Kat & Ana. Thought Kat & Ana would probably be a nightmare to code as far as PM is concerned.
Mona could throw pizzas at people, it'd be great!
Kat and Ana would be trouble, but it's possible. I've been looking around Kitty Corp a lot today, and I found some posts by Dantarion from last month in a Clone Engine thread where he says that he can port them with a bit more difficulty. The site's having 503's right now, but he posted pictures of an Ice-Climber-paired version of Link, Marth and Jigglypuff, with the Marth and Jiggz ones being ported over DK if the battle portraits are anything to go by.
Actually, I just remembered a reddit page in my history where he posted the same ones.
Here's the reddit page where he posted. (It was one to announce a live stream of him cloning a character, the pics are in a comment a little bit down.)
http://www.reddit.com/r/SSBPM/comments/1qpff0/clone_engine_live_9_pmish/
And the Kitty Corp thread:
http://forums.kc-mm.com/index.php?topic=64101.45
In case you get 503's when trying to check Kitty Corp or don't want to read through the posts (the thread's kind of heated at some points, not pleasant to read through honestly), here's the relevant comment.
Dantarion said:
It is definitely possible to port ANY character in this game. This includes IC's, this includes transforming chars. Its a bit more difficult, but it is.
Having said all that, I don't think they're likely and there are probably better characters.