We recommend you to take the Thai Government direct bus that departures everyday from Mochit 2 Bus Station at around 9:00 am. instead of the buses offered in Khao San Road, because these “direct” buses from Bangkok to Siem Reap, are part of a multi-stage scam, where visitors are tricked into paying extra for the visa, and exchanging their money at a poor rate. They contain many delays, making the trip last for as much as 12 hours. If you find yourself in one of these scams, cut your losses and find your own way throught the border and Siem Reap via minibus. The shuttle bus from right after the border, and even the bus station it leads to, is all a part of the scam.
The Thai Government bus company provides a direct bus from Bangkok to Siem Reap, which you can book the ticket directly at the Mochit 2 Bus Station, on the first floor.
You can also call to +66 2872 1777 or if you are already using thai sim card: 02 872 1777 you will get the payment code, and you can pay for the booking at 7-11 convenience store (everywhere in thailand), then keep the receipt to get the real ticket at the bus station on the departure date. It costs about 750 Baht/ticket and the bus leaves daily from bangkok at 9:00 AM in the morning and from Siem Reap back to Bangkok, the bus leaves at 8:00 AM in the morning.
This option is the best way to avoid all the scam at the border, generally, the tourists who buy the ticket from agencies on Khao San Road have to change the bus at the border and got asked to pay more for some other stuffs. While the government bus has the priority at the immigration board, and if you have some problem with your visas, the staff of the bus will help you figure it out. At first, you got to get off at the office where they arrange the visa for you (except thai passengers) the bus stops here about 20 mins, and they also serve the passengers a meal box in the bus. Then, 8-10 mins after every passengers have to get off at the Thai immigration office to get departure stamped, and then you have to walk to the Cambodian side to get the arrival stamp, that’s where the bus will wait for you.
The whole trip takes 8-9 hours to get to Siem Reap, takes about 4-5 hours to get to the border. You can buy the visa at the border paying with US Dollars.
Besides the BKK-SR route, they also provide the route from Bangkok to Phnom Penh, which costs 900 Baht, the bus leaves at about 8 AM in the morning. For other international routes, there is a bus provided from Bangkok to Vientiane, Laos, and Pak Se as well.