For Bangkok I'd recommend the Dusit Thani. Dusit are a Thai owned hotel chain (various level hotels but the Thani brand are the luxury ones) and this hotel is lovely. Treat yourselves and upgrade to the club floor for extra luxury. Hotel is in great location on the Silom Road.
We've stayed there twice and will go back. Loads of international hotels in BK, but a Sheraton is always a Sheraton, ditto Hilton, Westin etc. These hotels are luxury but part of their lure is that you always get the same basics.
Haven't been to the islands, we've done Hua Hin (loved it) and Pattaya (hated it) as well as Chiang Mai. Stayed in Dusits all the way. There's a gorgeous looking Dusit Thani in Phuket, and they have villas as an option.
It's worth emailing them to see if they'll give you an even better deal if you stay in Dusits all the way. We saved an extra 20% by doing that!