Hm...Hard to tell because I like a lot, but if you ask about the best ones, here is my "small" list :P
Da Capo ( Nemu-chan ), Kimi ga Nozomu Eien ( Haruka-chan ), Elfen Lied ( Nyu-chan, Lucy ), Air ( Misuzu-chan ), Clannad ( Kyou-chan, Tomoyo-chan ), Sword Art Online ( Asuna-chan ), Amagami ( Haruka-senpai, Ayatsuji-chan )...etc