Best prices? Best times. More comfortable seats?
Best is subjective and vague
What country's passport are you travelling with ?
Options are Megabus, Greyhound, and Amtrak. They all have their advantages and disadvantages with respect to price, schedule, and comfort.
Train is much more comfortable, but takes a long time at the border crossing and is prone to delays along the route.
Buses are less comfortable, but quicker at the border and only delayed by traffic issues.
As a citizen of the UK, upon entering the US, you will be required to obtain an I-94W Visa Waiver, in addition to having your passport. To obtain the waiver, you will be photographed, fingerprinted, and pay a fee of $6 US.
The I-94W visa waiver will be “electronically” attached to your passport.
The process of obtaining the I-94W waiver takes on average ½ an hour.