It really depends on your interest.
Shopping, sight seeing, history. big city, Thai lifestyle = Bangkok
Beaches, resorts, activities, fun and frivolity = Phuket
Culture, History, quieter lifestyle, country life = Sukhothai.
Phuket is also OK for shopping but a tad more expensive than BKK. BKK has a lot to offer but it is big crowded city.
Your pick. Do some research thoroughly before you decide.
Cheers