I can highly recommend the JW Marriott in Sukhumvit Soi 2. They have a great deal at the moment on Wotif (Wot hotel - it's supposed to be a secret) for 3500 baht a night including taxes and charges. You can upgrade to the executive floor and lounge for an extra 1500++ a day.
http://www.wotif.com/hotel/View?hote...5662&startDay=