Silom and Sathorn districts are upscale, well-connected, and close to the river — excellent for business and mid-range travelers. Sukhumvit is the expat and tourist hub: Nana and Asok areas for budget/mid-range, Thonglor and Ekkamai for trendier local-facing hotels and restaurants. Khao San Road is the backpacker zone — good value, chaotic, not very 'Bangkok.'
Bangkok's mid-range hotel value is extraordinary: $40-80 buys a proper hotel with pool that would cost $200+ in London. The Lebua State Tower (where the Hangover Part II was filmed) and Mandarin Oriental (consistently ranked among the world's best) define the luxury end. Stay near a BTS station — traffic makes non-skytrain areas much less convenient.