My favorites are actually #3 & #4 but, hey, that's coming from my motocross (the two-wheel variety) background.
I like standing at the back of a turn, taking pictures of the riders coming right into the camera. All the better if other riders are in the same shot. Also, if the track has turns with some loam, there is nothing better than a shot of the rider kicking up a big roost exiting the turn. Great from the front or the rear!
With "motocross" tracks turning more and more into "supercross" ones, IMO, too many photographers focus on jump shots. I like the above, first-turn pictures and those in the pits showing what all goes on behind the scenes.
Take care.
HrcRacing