the most important component is the steering wheel. because wit out that you ant goin no where. unless you got vicgrips.  

naaa!  

WOO HOO! YA HIT IT RIGHT ON THE HEAD! BUT BONUS QUESTION,WHY?  

mr Mculloch ya got the nail right on the head! D'ya know why??
cause their the only part of your rig which touches the ground,therefore,they provide the only tractive force,everything else is just there to make them move,,,everything,motor drivetrain everything,so the future of four by will be finding the way to maximise the way a tire interacts with terrain.The tractive force maximization,any nerds w/ a laptop and engineering in their blood will make a bundle telling the mfg who wants a algorithom to handle mud and rocks