Chidlom / Ploenchit / Withayu road or Siam, all walking, taxi, BTS, mocy short distances. If budget isn't an issue don't bother with the cheaper expat areas because they're only cheaper because they're further away from the CBD. Best guidance will come from members that have lived here in BKK for awhile rather than tourists and itinerents.
Soi Rumrudee has lots of decent places to rent and its easy to walk through the All Seasons complex onto Withayu (Wireless) Rd. which is where the US Embassy and many other large office buildings are situated.