Four Seasons without a doubt. I was last November in Bangkok, tried some 5 stars hotels. Capella Bangkok, Rosewood, 137 Pillars suites but without a doubt Four Seasons is a clear winner.
Also stayed at S 15 Sukhumvit, nice also with a location hard to beat for a budget hotel.
Next trip will stay at S 15 again, at Mercure at Soi 11, at Park Hyatt and again at Four Seasons.