depends on the activities...
if at the beach or for just relaxing at the resort, 5 star is definitely preferred

if activities are mainly outside the hotel, sightseeing, visiting this and that or hopping through the country from city to city, 3-4 stars are sufficient....actually many remote cities in Thailand do not have any 5 star property...the only 4 star in town is usually an old fashioned huge but run down dark and boring hotel, owned by the local Godfather, which I try to avoid...

global chain is not necessary for me....I probably stay more at global chains that I really want due to points collection and status requalification....if the Thai chains would be a bit more professional re their membership programs, more expats would stay there