I think Songkran in Chiang Mai is definitely the best and the wettest in Thailand. Tens and thousands of men, women and children flood the streets with their squirt guns. I wish I was here every year as a child. The buckets of ice cold water from them is a welcome respite to the summer heat in April. The never ending flow of water on each and everyone, endless party on the streets, loud blaring music and street dance makes Chiang Mai one of the best places to enjoy this festival.
Songkran in Phuket is an ultimate street dance party with tons of ice cold water and what more, the beach is right next to it. Patong beach is already well known for its night life and raucous scene. It doesn't take a back seat when it comes to one of Thailand's favourite festivals. Well, If you want to try and have a look at one of the beauty pageants in Thailand, head to patong beach. Experience their miss songkran beauty pageant along with a beautiful water parade and crazy party scenes.
Thailand's previous capital has its own royal style to celebrate this traditional festival. You can experience the traditional alms giving ceremony to 999 monks at the Wihan Phra Mongkhon Bophit to water splashing with none other than the mighty elephants. Finally, Thailand's culture is truly experienced in this old heritage city.
If you are heading to the south, other than Phuket you can also visit Koh Samui during the festival. The people here are quite serious about their water fights. With a mix of natives and tourists, this place is all in all fun to be.
Of course, the current Thailand capital is not going to take a backseat when it comes to their national festival. It has street parades to water fights to crazy parties all day and all night long.