Vurt some of my favourite groups that I lean towards are:
Anime-Keep - probably my fav - did an excellent job with Avenger, Hundred Stories, Shingetsutan Tsukihime, Scrapped Princess, Full Metal Panic and also current series Full Metal Alchemist and Monster.
ANBU who do alot of joint projects with Anime-Keep such as FMA and Naruto with AnimeOne. Also picked up a great new series called Madlax based of an excellent manga!
AnimeOne Very good group that did an excellent job on Ghost in the Shell series before they dropped it due to liscencing (Laughing Man Fansubs finished the series though and did a good job). I'm currently following them for Bakuretsu Tenshi, Galaxy Railways (joint project with Live Evil),
Ani-Kraze Another fav of mine who did an excellent job on Dragon Drive. Following them for Paranoia Agent, Yugo Negotiator, Gungrave.
Other notable groups are Live Evil (doing an excellent job subbing Condor Hero), Anime Kingdom (although I think they've been a little dead of late), Anime-Cookies, We Suck, Anime-Empire, Elite Fansubs, Lunar, Ricebox (although they disbanded, but were legendary for their small encode sizes and high quality, such as Witch Hunter Robin), Soldats, The Triad.
Of course there is heaps more, but these are the ones I look out for when it comes to unlicenced series. Then there is the dark world of Baka Updates that lists groups doing licenced series and there are countless groups doing DVD rips.