From the bus station in Bangkok "Morchit" to the bus station in Chiang Mai "Arcade Bus Terminal" there are daily buses. At about 520 baht and a journey time of 9-10 hours, it is an acceptable transport option. The advantages are, little can go wrong. All you have to do is make it to the "Morchit" bus station, buy a ticket directly there and get on. But to get the more comfortable seats, it is advisable to buy the tickets online in advance. At least one day in advance, because there are only a few "first class" seats per bus. As in all of Thailand, the air conditioning is turned up full blast on the bus. Especially uncomfortable on the feet. The first cold is almost certain after a 10-hour bus ride. It is worth taking long clothes or a thin sleeping blanket. Something for your head and ears is also in order. A hoodie is handy in case you get blown directly by the cold wind of the air-conditioning system. Depending on where you sit, it can be very uncomfortable after 5 minutes.
In summary: The only advantage of the bus is the low price.

There are about 5 trains a day that go directly to Chiang Mai.
The train starts at the main station in Bangkok "Hua Lamphong". The train journey takes about 12 hours and costs 600 Baht in first class. The second class is only 281 Baht for half price. Such a train journey is a bit more comfortable. Less curvy and bumpy roads and a steady speed without stops at traffic lights or traffic jams.

The train is preferred by the majority for a journey and especially the night train (sleeping car). It is recommended to book a 1st class sleeping compartment for about 1250 Baht. If you want your own compartment, a ticket in the sleeping compartment costs 1953 baht. But beware, the sleeping cars are very popular. You will most likely have to book a ticket at least one week in advance.

The best solution, however, because it is the fastest, is by plane. The flight takes just 1 hour. Of course, you still have to take into account the journey of about 1 hour. And the 1 (better 2) hours you have to be at the airport beforehand. So a total of about 4 hours. Depending on the season and flight provider, as well as the time and day of the week of departure, flight tickets start at around 600 baht (approx. 16€). However, you still have to include the taxi (e.g. Grab) or the bus to the airport.

Bus to the airport: approx. 3€, 60 minutes
Taxi to the airport: approx. 20€, 60 minutes

The taxi to the airport is the most expensive but easiest solution for getting to the airport. Picking up at the hotel is also stress-free. However, if you have enough time and dont have heavy suitcases, you can take one of the many buses. You can book a taxi one day in the many small travel agencies in Thailand or directly online on 12goasia. If you have enough time, you can also order a taxi in the morning before departure using the Grab app. I recommend the travel agency because it is the easiest. You can also book your flight there if you dont want to do it yourself online.

Flights depart from the smaller airport "Don Mueang" and from the large international airport "Suvarnabhumi". Be sure to check which airport you are departing from. Depending on traffic, the airports are 1 hour apart. Therefore, the time from the hotel to the airport should be well calculated (including traffic jams). At certain times of the day (after-work traffic) there is no way through Bangkoks city centre.