I'd go with a heavy pomade. Oil-based pomades don't harden up - which is good because you still won't get out of recombing your hair after removing your helmet. There's no product that can just ignore the pressure a good helmet puts on your hair. The key is to chose a product that easily lets you recomb your hair and to have that comb in your pocket whenever you feel you'll need it.
I'd suggest the Uppercut Deluxe Monster Hold. The hold is very good and it's not very greasy. My second suggestion would be Dax Wave & Groom. It's cheap, holds like hell, but is also greasier than the Uppercut Deluxe.
Water-based pomades will wash out easier, but they'll also harden up and make recombing your hair more difficult.
The key to finding a good product is trying them on your hair, after doing your "thing" (washing, blowdrying etc). Reviews can be a good guideline, but will never get you out of experiencing the stuff stuff does to your hair yourself.