If you like the "kick" I'd recommend either a 15" or a 12" in a sealed enclosure. Some people may disagree with this, but it is really personal preference. A decent 15" in a minimum-sized sealed box will produce a significant punch(better than a 12" or a 10" IMO) when properly positioned.

Most people don't understand that the position and firing direction of the sub is extremely important when designing and installing any sound system. When it comes to vehicles, acoustics are never same. All cars are different, usually have several materials inside the cab aswell as many variables like windows(changes air pressure and adds noise. Setups where air space limited, like trucks or coupes, tend to sound louder and "fatter" with the windows opened).

So just experiment, really. Experimenting is only limited by box dimensions as some of the way it may sound the best it won't necessarily fit. Size is always the culprit in car audio, so sacrifices have to be made sometimes.

You may want to hook up with one of the car audio places on here like P&R or MCOR.