I think the tropics during the Monsoon season. Especially when it rains late in the afternoon, clearing just before the sun sets. You get the most amazing cloud patterns and colours and the light has a real intensity. Definitely worth putting up with the high temperatures and humidity!
I'd have to agree with Sander also. Lake Tekapo in New Zealand is pretty amazing.
And last but not least, the Mediterranean. The Greek Islands, Venice, South of France - the light is always fantastic.
I'd say the skies in Africa really add a certain "magic" to lots of pics.
I don't think it really matters where you go. There are always great spots to take nice shots. (even you own back yard)
You can see some of my shots (not verry resent) at www.ghpsite.com For you travelers the low lights shots will be interesting
Iceland has the most amazing crystal clear air, and the landscape is just surreal. I'm at work right now--no access to the pics! I can't wait to go back and spend more time.
Barcelona for its unique Gaudi art and having mountain and see in the same place, like Rio but safer. Lovely mediterranean design is felt everywhere.
And great accommodation.
City's web site http://www.bcn.es
Although I think Time of Day is a more important vairable than the place. Mornings and late-afternoons always seem to have the best colors. The sun has to travel through more atmosphere when it is lower in the sky and that acts like a giant filter. Also light cloud cover also produces nice colors and you don't have to deal with harsh shadows (the filtering effect of the light cloud cover is the similar to early and late in the day), also you can then get nice colors in mid-afternoon.
That being said, and just judging from my personal experience from looking at my own pictures, I seem to get the best, most beautiful colors at high altitude (especially in Bhutan and Tibet). I replace my UV filter with a Haze 2A filter (which helps at high altitude because there are more UV rays up there). That has always worked well for me.
If I ever get around to scanning some of my pics I'll will certainly post some.