i really cant decide but here a list of my favourites (no sepcific order):
dr. dre
dj premier
mr. porter
j dilla
timbaland
rza
eminem
mathmetics
q-tip
dj muggs
hi-tek
dj khalil
focus
there are some more though. and i must say that ever since timbaland has gone into "superproducer" status producing almost every single in the charts as of late, i think his production is way too repetitive, predictable, mediocre and no where near what it used to be. he's a lot more diverse now in the sense that he is making beats for rock, pop, r&b and hip hop.