At my local nitro track (near Los Angeles, CA):
The main 3:
Losi (by a decent margin)
AE
Hot Bodies
Mugen - I see sprinkles of them now and then, and more at bigger races
O'Donnell - I only see mine and maybe when Barry Baker runs on a big race, otherwise, never seen another one
Kyosho - Only see them at big races...only about 2-3 entrants
Xray - almost never see