Excel Excel (from Excel Saga)
Not only is she incredibly sexy, but she's also extremely energetic. Energetic to the point that she would be annoying in a 'real' relationship, but as part of an orgy, she would be ideal. ... The real question is whether I'd be able to handle her or not.
Hyatt (from Excel Saga)
She and Excel would work great together as a team, and plus... her personality is just too adorable to resist. It may be cliche, but damn, I love it. It's very rare to see such a level of cuteness and innocence attached to a 'mature' character. Which is great for guys like me who sometimes prefer their anime women to be a little more 'womanly'.
Misato (from Evangelion)
Somehow she just seems as if she would enjoy it. She's very laid back, so it would keep things light and fun. Unless of course she's in one of her 'oh god an Angel is attacking mood', in which case, not so much. She's still incredibly sexy, and it's great to have a thirty something in on the action. All of that experience with Kaji will pay off.
Faye Valentine (from Cowboy Bebop)
Faye has the best body of all my choices. Misato and Hyatt I would want to do just because I love them so much. Faye I'd want to do just because she's incredibly hot. A sex bomb if I ever saw one.
Kagami (from Lucky Star)
This one is a bit of an odd duck compared to the others, and is based more on 'fan art' than anything else. I love the character of Kagami on the whole, but within the context of the show Lucky Star I never really considered her attractive in any way. It wasn't until browsing around Photobucket and finding some original art that I thought to myself: 'Damn... Kagami can be hot.' It would be nice to have someone fresh out of high school anyways. Add a little bit of variety to the proceedings. Some young blood.