West towards Poipet/Thailand, Battambang.
After about 100 kms, at a city called Sisophon, there is a junction for the roads going to Poipet, border with Thailand, and Battambang (it's 80 kms more to Battambang). From Siem Reap, the road to Poipet/Thailand and Battambang is the road to the airport (13 kms away for Siem Reap), and there are plenty of luxury hotels along the way for several kilometers. It means that most of the vehicles on this road are going either to a hotel or to the airport. However hitchhiking is easy because there are enough cars going to the border of Thailand. A written sign can be a good idea to let drivers know that you want to go further the airport, just to avoid to have cars/tuk-tuk/motorbike stopping every minute for nothing.