Well my favorite's Nagi. Didn't like her doing Al so much, though she was better than the english version of Al.
Others, Shana, Louise etc I think she does a really good job on, and obviously I'm a fan ^^.
Probably Shana, it's the first place I heard her, she's also the first seiyuu I could recognize by hearing. :D Now I go out of my way to watch shows just because she's in them. :D Although maybe Rizel comes in a close second because well, she sang both the OP and ED for that show.
Apart from the three major roles listed by ChiBuki, I really liked her Elric Alphonse. Her tsundere roles all sound fairly similar, and I certainly do enjoy that voice, but her Alphonse shows just how good of a seiyuu she really is!
Ermmm while her tsundere roles were really cute I have to say Kagura gets my vote. Of course I haven't seem all the anime series with her in it, but Kagura is something different from a lot of her other (<--warning: based on the shows I've watched b/c Alphonse and Hotaru were definitely not tsunederes) roles and while I cannot get a good opinion about her acting, Kagura, or Kugimiya Rie's straightforward and *cough* sometimes stupid (character) made me laugh quite a lot...nah, her other roles were really good, but I do have a special bond with that show you see..;0