The closer to Asoke the better I think. I have a great condo in Phra Khanong and everyday I wish I was just a few BTS stops closer to Thong lo/Nana etc. taxis are great in the evening but during the day BTS is the only way to travel. Traffic makes daytime taxi rides torture.
If you need a hand I know a great Agent to help you find a place. Perfect English and French (and beautiful too).
One of my friends tell me, On Nut is the new up and coming area to get a trendy apartment.