Depends on where you want to visit, what you wanna do.
There are three main areas people visit...
1. Chang Mai is the northern mountainous region, temples, jungles, working elephants
2. Bangkok is Bangkok
3. Island areas...Phuket is the most visited island. Tons of hotels, resorts, and tourists. Krabi and koh phi phi are islands off the coast of Phuket with more picturesque beaches.
Koh samui is the second most visited island, more low key, cheaper, and nearby islands koh phagnan(has world famous full and half moon parties) and koh tao (best snorkeling and diving in the world)
I was there a week and only went to Samui and Koh Tao. Best island vacation I’ve been on