No countdown, the only two types of things that could possibly have a countdown I've been on are Cliffhanger and Velocity.
Cliffhanger would suck without a countdown as I love how its in the middle of the North Yorkshire moors you can be having a great view of the scenery and the rest of the park but BAM! before you know it your a few feet away from the ground!
Velocity, same thing, I love the way it edges you out of the station and all you can see in front of you is the huge hill, and you just sit there waiting, thinking, waiting, and then your over it.
No Countdown's provide more of a shock which is better IMO