How can I see the best scenery on the way? What buses should I take where?
You could get the bus, cross the boarder and head to Siem Reap. This would take an awfully long time and is not the most comfiest way to travel. My suggestion would be to fly - its about an hour - from BKK to SR direct and then spend the extra time exploring around SR. Its a lovely place to base yourself and a lot to see and do.
Bangkok Airways do a travel pass which is priced on sectors and we found this to be great value and very flexible. about 6 years ago
You can take the bus all the way from Bangkok to Siem Reap. There is just one way to go--through Poipet / Aranyaprathet crossing and then on National Road 6 to Siem Reap. It is a very, very long trip.
A few things to remember--most buses will drop you in Aranyaprathet and then you will need to take a tuk-tuk the border. You'll be met by Cambodian officials who will try to overcharge you for your visa while still on the Thai side. Someone will walk you across the border, and then you will be told you need to change money (you don't--US$ is accepted everywhere, and baht most places--but the exchange rate is 4000 Riel to the dollar--they will give you 1/2 that at the border) and then you will also be told you have to take an overpriced government private car or bus the rest of the way. Also, the border often closes at 5PM. So, keep your wits about you and be ready to be uncomfortable, to refuse to overpay officials, and to look around for your own transport.
If you are really up for an adventure, I suggest you stop in Sisaphon and go see Bantaey Chmar, but be forewarned there are no tourists there and even finding someone who speaks English or can take you there (on the back of a motorcycle for a couple hours on a dirt road) can be a challenge, but it will let you break up the trip. There isn't much to see in Poipet except casinos. about 6 years ago