The basic Settlers is my favorite, but my friends and I have been playing a lot of Cities & Knights lately, too. Totally different style of game with a ton of new strategies, though it takes more like 2-3 hours instead of the 60-90 minutes of a typical Catan game. I think Cities with 6 players would be crazy. 6-player Catan is already pretty intense.
A few months ago, I went to a test run of a new scenario that a few environmental science dudes put together. Mayfair's released it for free now:
http://www.oilsprings.catan.com/. Kinda silly, but worth a go if you like Cities and Knights, as a lot of modifications are similar. This is basically the greatest Catan mod of EricMM's life.