There are at least three companies that do a Siem Reap to Sihanoukville route but from talking with other travelers the best one is the $12 night sleeper.
This bus had air con, WiFi, nobody sitting (lying) next to me so I could stretch out. We did stop once for food and a smoke which helped me sleep till we arrived in Sihanoukville, a $4 tuk tuk (3 people) and we were at sea horse hostel. $10 for a fan room which is often subject to power outages but otherwise in a good spot with friendly staff.
