I personally like the Siri Sathorn. It's an apartment/suites type place run by the same management company as the Sukhothai Hotel. They have a pool (never used it but it looks decent enough). Free wifi. Full kitchen. Fairly standard breakfast offerings. The walk to Saladaeng BTS or Lumpini MRT is about 10 minutes. Plus they offer a free shuttle van to the Silom area or perhaps even further afield.

Looks like best rates they are offering on their website are Bt3300 ++. I find it to be very comfortable and the service quite pleasant.

There is a place directly across the street called the Bandara Suites (I think) which is supposed to be pretty good but a notch below Siri Sathorn however I think that also means it is cheaper.