I have to be honest here and say there are "no best classes" because all classes have weaknesses and strongholds to other classes. like I find Necrons are weak against eldar. because eldar have superior speed and can just make a fast regroup before the necrons will never reach you. or throw the necrons around with a few explosions. well you can allready say nite nite :P
tho I find Space Marine the base of the game. they have no real weaknesses but no real strengths either. Orks as allready said weak to stealth is pretty much true having 4 spacemarine scout teams with fully upgraded sniper can take out 8-16 orks in a blink of an eye and then dissipear again!
Else I never had any luck with Tau vs Orks either. where I find the Tau superior if they use all their fancy stuff. which the orks lacks and is maybe their biggest weakness! straight ahead and no tactical matter.
also seen some really nice tactics with longrange spacesister tanks with invisible morons in the front! lost to that tactic all the time using orks, spacemarine, chaos. but manage to pown that tactic a few times with Tau and Nekro, since nekro in close combat is invinsible and they have their lords which can takeover enemy tanks witha click of a button