Bangkok IMO only for those like lots of people, lots of pollution, traffic jams, good night life.
Pattaya is IMO like a smaller, cheaper version of Bkk.
Chiang Mai quieter, more polluted certain times of year, far from the coast, best scenery options, pathetic night life, worst public transport options.
All three have similar facilities, traffic problems and the main difference IMO is cost of living in them.