Well just about everybody swears by Valentine 1, except me.

I like the Passport Solo which is cordless, that way you can switch from car to car, no ugly cord, and you don't have to tear your interior up to bury it. It might not be quite as good as the V1, but the trade off is worth it to me, maybe I'll change my mind if I get stopped, we'll see.
These things are a must.