It depends on what I want at the moment.
I like MTB to meet Nicke and other BM's and get a bit drunk, I like Tai Pan for its music and the girls, I liked a bar in Soi Seapearl (don't remember the name but it was the first one on the right) because its friendly staff and it opened at midday, but I think it's closed now.... and sometimes I like some little bars from Kamala or Nai Harn because it is quieter.
As it has been said, it is very subjective from one to the other... I would say the best bar is the place where you feel good at the moment
