What do you guys like to see in wood grain?

If using the supplied block is in the rules and you want to respect that rule you can simply use a veneer cover if you want a different wood grain. Leave ~1/4" of the original block for the frame.
 
Here's the second car I ever built for league racing. It never saw the track though. It's a BSA block with a Book matched Burled Walnut veneer top.

BurledWalnutCar_zps9278ce9d.jpg
 
I look for quarter sawed blocks. I like to go to hobby lobby or Michaels to inspect the revell blocks because they are in see through plastic. I look for the grain of the wood to be horizontal. Quarter sawn wood is more stable than rift sawn or plain sawn. I also bake my blocks at 225 for 4 hours then shut the oven off and leave them in there until cool.
Its my kiln process.