Hirano Aya is one of my favorite Seyiuu as well. She has a incredibly large and versatile vocal range. Not only does she do the voice for Haruhi but also Konata from Lucky Star and Garnet from Dragonaut. (If you've seen those 3 shows you'll know what I'm talking about)
My second favorite Seyiuu would have to be Horie Yui because she has such a ridiculously cute voice and does the voices for equally cute characters like Ayu(Kanon), Hanyu(Higurashi), Tohru(Fruits Basket), Siesta(Zero no Tsukaima), Suzu(Nagasarete Airantou).
And lastly, Kugimiya, Rie, or otherwise known as the Tsundere Seyiuu(No, not really) she provides the voice for 3 out of the very limited amount of Tsunderes I like. Nagi(Hayate no Gotoku), Shana(Shakugan no Shana), and Louise(Zero no Tsukaima).
A few others that have caught my eye, but I wouldn't call my favorites...
Ono Daisuke Yukito(Air), Hosaka(Minami-Ke), Itsuki(Haruhi no Yuutsu), Himself(Lucky Star), Jin(Dragonaut)
Fukui, Yukari Nono(Diebuster), Nia(Gurren Lagann)
Sugita, Tomokazu Yuuichi(Kanon), Rin(Shuffle), Soldier Blue(Toward the Terra), Kyon(Haruhi no Yuutsu), Hideki(Chobits)