To reach Siem Reap from the capital, you have many choices between the bus, taxi, plane or the boat.
1/ By land
To reach Siem Reap by land, you will need to travel on the national road 6 and travel 314 km.
The most economical way to reach Siem Reap is to travel by bus. You can opt for the company Capitol that charge between 6 and 15 USD per person. The price is very competitive, however, throughout the journey, you will see cambodian people get in and off the bus to make the journey more profitable for the company. The duration of the journey can last more than 6 hours.
For a more luxuous choice, opt for the mini van with Mékong Express (13$) or the bus with Giant Ibis with day and night shifts (15$). You can then reach Siem Reap at 6am in the morning.
You can also decide to rent a private taxi from 75$. If you are travelling with 3 other people, it will cost you 18,75$ per person. You can arrive at Siem Reap in 5 hours approximately.