When to go where in South East Asia

Thailand
Thailand is a very big country so the best time to visit varies slightly from the north to south. Generally speaking November - April will be drier and hot, often 32-36 degrees with January - March being the driest. The "wet season" is March - October, but don't let that put you off! It usually just rains for a very short time and often at night, which can feel really nice and refreshing in the otherwise hot and sunny day. There are fewer crowds in May-September if you want to visit tourist attractions without queuing for hours, this might be the best time to go. Temperatures still go up to lower 30s.
Northern Thailand, in places like Chiang Mai, can get as cool as 5 degrees in November - March. This can be great if you're going trekking, but if you'd still like warmer temperature go in March-May before the wet season starts.
Bali
Bali and the rest of Indonesia is great if you have some time off during the summer months! Unlike most of the rest of South East Asia the peak travel months are June - August. This is when it's the driest and sunniest with temperatures around 25-30. Bali gets very busy this time of year so if you don't love crowds we recommend going around September or late spring (April - May).
Vietnam
Generally speaking the best time to visit Vietnam will be in the spring (February - April) and autumn (August - October). Of course, this will vary slightly depending on where you're going in the country.
The north, Sapa, for example, is best to visit in March, April if you like warmer temperatures, and October, November for cooler trekks but still with clear skies. December - February can get as cold as freezing and July - August are very wet. Halong Bay is the driest and warmest in March-May, ideal for kayaking. Ho Chi Mihn City, because of its proximity to the equator, is ideal year-round, and Hanoi is driest and warmest in October - November.
Philippines
Peak season is when it's the driest, November - April. The shoulder months of October and May can actually be ideal for fewer crowds and still very warm weather. But the tropical climate means the weather can be unpredictable and is usually hot and humid year-round.
Myanmar
You can visit pretty much all year round but we'd stay clear of July - September when it's monsoon season and they even close some beaches. Winter months are the nicest (November - February) and March to May can get uncomfortably hot.
Cambodia
Peak season is November to March again. Of course, this means it can get very busy so you might want to go in the shoulder months of April and October. The wet season in the summer is still a great time to visit. There might be rainfall in the afternoon but the rest of the day is nice and sunny, and the greenery is spectacular. The rain rarely affects any tourist attractions.