A lot depends what sort of thing you want to do while you're there. Sukhumvit area is more... I don't want to say "Westernized" but it's definitely more "global". If you're looking for reasonably high-end or non-Thai bars and restaurants, that area's probably your safest bet.
If you want somewhere that still feels a bit more Thai without people looking surprised when you walk into a 7-11, then maybe Bang Rak area (near Saphan Taksin BTS station) would be a good shout. Still has some swanky bars but a lot more of a real Thailand feel, IMO, and there are a few hotels on the river near the station.
If you want to wander around and really get a feel for the older parts of BKK, then somewhere over in Thonburi might be for you. That's more of a daytime thing though (no idea if you're there for work or passing through on holiday).
Honestly, the first thing I look at for accomodation in BKK is how close it is to a BTS or MRT station. Nothing worse than starting your day, or evening, with a sweaty half-mile walk! That is assuming, of course, that you won't have a personal driver during your stay