hmm a threesome with Cagalli and lacus for sure, though i'd fuck the hell out of lacus from gundam seed
http://fc04.deviantart.net/fs18/f/2007/155/c/1/L_x_C_ID_by_Lacus_x_Cagalli_Club.jpg who wouldn't jump in on that?
as for other the list goes such as
3)Rei Ayanami [evangelion]
4)Sakura [cardcaptor sakura]
5)flay [gundam seed]
6)peach [mario]
7)seras victoria [hellsing]
8)Miku Hatsune [vocaloid]
9)lum [urusai yatsura] (can't remember how to spell that but ya get the idea)
10)Mizuho Kazami [onegai teacher]
The last anime character I can actually think of willfully searching doujin hentai of was Tsunade from naruto, and that was quite a long time ago (couple years I think).
Let's see... there's Arcueid (Tsukihime), Hisui (Tsukihime), Saber (Fate/Stay Night), Tohsaka Rin (Fate/Stay Night), Aisha (Koihime Musou), Sei (Koihime Musou), Karin aka. Sousou (Koihime Musou), Shouko (Baka to test), Kujikawa Rise (Persona 4) and many more, I can't mention them all.
Here are the top three of my list :
1. Satomura Momiji (Fortissimo Akkord Bsusvier)
2. Yukikaze Panettone (Dog Days)
3. Nagiza Suzushiro (Fortissimo Akkord Bsusvier)
Not anime but i'll do Samus and Peach (same time LOL) as for anime Morrigan (yes i don't care if i die doing her), Dark Magician Girl and too way many... :D