Mark Copplestone
www.copplestonecastings.co.uk
Did stuff for Foundry, GW, Grenadier UK
Steve Saleh - ex Foundry
www.gorgon-studios.com
Mark Sims - ex Foundry
www.crusaderminiatures.com
I love these three for making easy to paint rank and file figures. The command figures are also excellent. I think the other two owe a debt to Copplestone for setting the style/example. You could probably add Mike Owen to that list. Sort of reads like a list of great sculptors chased off by Foundry....
As far as sculptors who really set the stage for Citadel Miniatures/GW in particular, this is my opinion. (And this is more of a "historical" line up than a current list, I guess.)
Alan & Michael Perry. Especially the humans, dwarfs, and historicals. Plus their share of the chaos range.
Kev Adams. There were competing artists working on greenskins, like Bob Olly and Nick Lund (sculptors) and Ian Miller (illustrator) but only one was known as "The Goblinmaster." Subsequent artists have built on his sculpts/designs, but the look is distinctly his, and to his credit the influence has moved beyond just Citadel/GW. I believe he also did some work on the old chaos range as well.
Jes Goodwin. The other big influence on the more "fantasy" races, high elves, wood elves, skaven and 40k Eldar. Also did some of the old chaos figures. Plus I believe he has been responsible for a lot of the general artistic direction/management at Citadel/GW in general.
Those are the big names I think of. But also there's...
Trish & Aly Morrison. Chaos & monsters. Plus most of the Marauder range. Some undead an a good portion of the pre-big-hat chaos dwarfs, too.
And... Nick Bibby, Dave Andrews, Mark Copplestone, Tom Meier, Bob Naismith... more that I'm sure I'm missing. At this moment I can not remember the name of the sculptor that did a lot of the Citadel and Foundry artillery. Obviously under appreciated! In later days, I think Chris Fitzpatrick (as someone else mentioned above) really revived the dark elf range and thus influenced what other sculptors did with the Dark Eldar as well.