The Regent is now known as The FOur Seasons.

Mandarin Oriental are also due to open up a place soon near Chiang Mai.

I stayed at the Regent in Chiang Mai in '98, before it was sold.
It was one of the most beautiful places I have ever seen, the view of the Mae Rim valley is breaktaking. I still think of that view...

IMO in Siem Reap options should be the Raffles properties or if you like boutique properties and want to splurge Amansara, else boutiquey and more reasonable Pansea or the Shinta Mani. The Sofitel is a typical large hotel which could be anywhere. The Raffles has a huge amount of history attached to it from the moment you step in. I have earlier compared all these options in a post but will recap here again. Raffles is great if you like large hotels with hustle, bustle, colonial character. Rooms are a little small. Amansara is the ultimate in boutique hotel luxury in true Aman style, only 12 rooms, provide the facility for you to see the temples by cycle rickshaw which in good weather can be interesting. Pull up my earlier posts on tyhe Pansea and Shinta Mani. In case u stay at Raffles def book a dinner at Amansara - it is across the road and you will enjoy a dekko.

We stayed at Amansara in S. Reap last fall (I think it had just opened). Very serene, very minimalist, very beautiful. (And very expensive!)