Thailand's bus network is the one that goes everywhere. This page lists 1,281 bookable journeys — more than every other mode in the country combined — from 42 separate starting points, so the question is usually which vehicle to take rather than whether a route exists at all. The shortest runs here are half-hour airport shuttles; the longest cross most of the country overnight.
One thing to check before booking, because our bus guide is blunt about it: "VIP" and "Sleeper" do not mean the same thing between operators. A standard coach seats forty to fifty with air conditioning and sometimes a toilet; a sleeper reclines much further; a minivan is faster on some routes and more cramped on others. The label on the listing is not a standard, so read what the vehicle actually is.
For the islands, the bus is usually only the first half. Combined bus-and-ferry products cover Bangkok to Koh Phangan and the Krabi-side crossings as a single booking, which is simpler to buy and no simpler to change: each stage can still carry its own operator and its own conditions.