For heading from Siem Reap to Bangkok (as in the title of your post), the quickest and cheapest way is probably via Poipet. The taxis tend to run quite a bit cheaper from Siem Reap to Poipet rather than the other way around. Once across the border and into Thailand, you might even be able to pick up a cheap taxi all the way into Bangkok - one that is trying to make gas money for the return trip back home.
You'll find good additional info on this option in the trip reports section and also in the features section. Give them both a look. Well worth your time.
Depending on your budget, you can also fly from SR to Bangkok as well. Cheers.