This is a tough choice...
Fairlight were the dominant group in 1993, releasing most majors and by then, A1200's were out so they tended to fix lots of memory allocation bugs in games. Their quality was also consistently very high and had some top guys like Gaston, N.O.M.A.D and Galahad.
Skid Row seemed to dominate from 1990 to 1992, with good old FFC doing most the work. Pretty consistent quality too and some nice intros.
Paradox seemed to always be around but not in the quantity of the other groups. They seemed to "sleep" for several months and came back with Desert Strike. Prior to that they did good work on Ivan Ironman Stewart's Super Offroad Racer (who can possibly forget that intro of the devil when they were at war with Angels! For anyone that hasn't seen it, grab the Paradox version of this game! Paradox seemed to be so full of themselves at times (from reading the Paradox website) saying that they beat all other groups into the ground (except Fairlight who outlasted them - even they admit that!) Blackhawk was a great Amiga cracker/coder.
Quartex cracked practically everything from 1988-1990 and then slowed down a lot after that. The early games (1988-1992) had lots of bugs on AGA machines and accelerated Amigas which was not Quartex' fault - at the time they didn't have 030's etc so the games worked. I personally admired them as they didn't add buggy intros and trainers to the games, just cracked them and spread. I have disassembled quite a few Quartex cracks and there are some really elegant ones where they didn't touch the original data, just "patch" around it - a difference on the original disk and the crack shows up minimal changes which is great. Not sure who wrote their poetry, always seemed to be someone on drugs writing that shit on their text intros. And they always asked you to purchase the original if you liked it, nice of them.
Finally Crystal seemed to be a major force around about 1990 and lasted a few years, they seemed to be abused a lot by other groups - not sure why! N.O.M.A.D was cracking a lot of stuff for them for a while and I can't remember too many dodgy cracks.
So to sum up, it's almost impossible for me to choose a favourite group but by a whisker I think Fairlight wins...
[Perhaps we should do a "worst cracking group" poll? That would get some interesting results I'm sure. There were quite a few lamer groups with dodgy cracks (or ones where on a password screen you would have to write "XXXX rules" as the password or it would crash) and those awful "AGA fixing" groups like Fix Ltd who couldn't fix the easiest game without making it crash on the majority of machines!]