Best way to get to Koh Samui from Bangkok, Thailand

Koh Samui is located 470km south of Bangkok. It's pretty easy to get to. Flight is generally the best and by far the most popular way to get to the island, but there are other slower options like bus and train (which necessitate a transfer to a ferry).

I visited Koh Samui when I travelled across Thailand in April 2019 (as part of my 4-month Southeast Asian trip), and did lots of research and planning for my trip. There are 3 options to get to Koh Samui from Bangkok:

Flight
Bus + ferry
Train + ferry

The best option is to fly because the other two options both take a significant amount of time. Flying is however the most expensive option.

Option 1) Flight (best option)
There are 18 daily direct flights from Bangkok Suvarnabhumi Airport (BKK) to Koh Samui, all operated by Bangkok Airways. Thai Airways also sell tickets on this route, but it's all codeshare flights operated by Bangkok Airways
Bangkok Airways is pretty very good airline. It currently has a 4/5 star on Tripadvisor. I've flown it numerous times within Thailand and really liked their overall service level
Fare: $90-$110 USD per passenger for a one-way ticket, and $190-$210 round trip
Fligh time: 1 hour 30 minutes each way
Flight schedule: the earliest flight departs at 6:00AM and the latest flight departs at 8:10PM, and there'a flight every hour or so
Departure airport: Suvarnabhumi Airport (code is BKK). Don Mueang (code DMK; it's Bangkok's other airport) does not have any flights to Koh Samui. You'll need to get there by taxi
Arrival airport: Koh Samui Airport (USM), which is the only airport on the island. You'll need to take a taxi to get to your final destination from that airport
How to book: since Bangkok Airways is the only airline flying this route, I suggest using their website (https://www.bangkokair.com) to book directly. Although you may want to double check a metasearch site like Skyscanner to see if other airlines have started operating this route

Option 2) Bus + ferry
A cheaper option than flying is to take a bus to either Chumphon or Surat Thani, and then take a ferry to Koh Samui. There's unfortunately no direct ferry from Bangkok to Koh Samui
Total price: $20-$40 USD per person each way
Total travel time: as short as 10 hours 40 minutes to as long as 17 hours 15 minutes. Most of the buses are overnight options that departs in the evening and arrives in the following morning
Departure point in Bangkok: varies depending on which bus and ferry company you choose, but all are in central Bangkok. Once you book your ticket, they will tell you exactly where to board the bus
Arrival pier in Koh Samui: the ferry arrives in either Nathon Pier, Lipa Noi Pier, or Maenam Pier
How to book: the best way is to book a combined ticket that has both the bus and the ferry legs rolled into one ticket, rather than booking the two legs separately yourself. You can book these kinds of tickets through Ferry Samui (https://www.ferrysamui.com)

Option 3) Train + ferry
The route is made up of train from Bangkok to Surat Thani, bus from Surat Thani to Donsak Pier, and then ferry from Donsak Pier to Koh Samui
Total price: $20USD - $40USD per person each way
Total travel time: 13-16 hours each way
Step 1: From Bangkok Hua Lamphong Station (located here), you can take a train to Surat Thani Train Station (located here)
Trains are all operated by State Railway of Thailand. There are two kinds of trains: Special Express trains that take ~9 hours, and other trains that take ~12 hours
Fare:
First class: 519 baht ($17USD)
Second class: 248 baht ($8.2USD)
Third class: 107 baht ($3.5USD)
There are both overnight and non-overnight trains available. If you're taking the overnight train, I highly recommend getting the first class as that's the sleeper class. If you're taking a day train, then second class is pretty doable as they're incline chairs. I don't recommend third class tickets unless you're on a very tight budget
There are 12 trains each day:
Train 43 - 8:05am departure from Bangkok -> 4:45pm arrival in Surat Thani
Train 171 - 1:00pm -> 12:22am next day
Train 31 - 2:45pm -> 1:23am
Train 45 and 37 - 3:10pm -> 2:00am
Train 169 - 3:35pm -> 3:45am
Train 83 - 5:05pm -> 4:24am
Train 173 - 5:35pm -> 5:48am
Train 167 - 6:30pm -> 6:23am
Train 85 - 7:30pm -> 7:06am
Train 41 and 39 - 10:50pm -> 8:05am
How to book: you can check the latest schedule, prices and book your ticket through the State Railway of Thailand official website: http://www.railway.co.th/main/index_en.html
Step 2: from Surat Thani Train Station, take a bus to Donsak Pier
There is a 2-hour bus ride from Surat Thani Train Station to Donsak Pier where all the ferries to Koh Samui depart from
The buses are operated by Phantip Travel (https://www.phantiptravel.com)
Price: 180 baht per person each way ($6USD)
How to book:
Go to Phantip's website and book on there
Note that when you book, you need to specify if you're going to Donsak Seatran or Donsak Raja. They are two separate piers located 10 minutes drive apart, serving two different ferry companies (Seatran and Raja, more on them below)
By the way, on Phantip's website you can book a ticket directly to Koh Samui (instead of to just Donsak Pier). If you book this, Phantip will take care of your tickets for both the bus to Donsak Pier, as well as the ferry from Donsak to Koh Samui. I believe they partner with Raja as their all-in-one ticket takes you to Nathon Pier on Koh Samui
Step 3: take a ferry from Donsak Pier
There are two ferry operators leaving from Donsak Pier to Koh Samui: Seatran and Raja. In fact, Donsak Pier actually is made up of 2 separate piers, one for Seatran (located here) and one for Raja (located here)
On Koh Samui side, Seatran arrives at Bangrak Pier (located here) and Raja arrives at Nathon Pier (located here). The two are located on opposite sides of Koh Samui so make sure you're going to the one closest to your final destination
Travel time: 1 hour and 30 minutes for both ferries
Price: 452 baht ($15USD) per passenger each way for Seatran, and 130 ($4.5USD) per passenger each way for Raja
Schedule:
Seatran: 2 daily departures at 11:00am and 2:30pm
Raja: 15 daily departures starting at 5am, with one ferry every hour after that (so at 6am, 7am, 8am, etc.) until the last ferry at 7:30pm (2nd last ferry is 6pm)
How to book: you can buy tickets directly at the pier, or book from the ferry's websites
Seatra: https://www.seatrandiscovery.com, Tripadvisor (rated 3.5/5)
Raja: https://www.rajaferryport.com, Tripadvisor (rated 2.5/5)