The Architectural Association on Bedford Square in London would be considered to be one of the best.
Nearly everybody of repute in the field of architecture has passed through its doors either as a teacher or a student: Zaha Hadid, Rem Koolhaus, Daniel Libeskind, Ron Herron, Nigel Coates, Tom Henegan . . . Archigram emerged from the AA . . . the list is endless.
As a student from another college and discipline, I worked on a joint project with AA students and in the process attended some riveting juries/crits that would go on late into the evening. I enjoyed it immensely precisely because they are immersed in concept.
As a priority, the emphasis is on experimental, philosophical, non-linear approaches to problem-solving and learning...
Pretencious (sic?) (moi?)
Yes... they do take themselves incredibly seriously and their, at times, willful obsfucation of the English language gets a bit tedious, but if it's a change of atmosphere you're looking for it'll be like a shot (of something good) in the arm.
It is tres expensive, and there is a touch of the 'finishing school for rich kids' vibe about it, but they offer scholarships, and as for the rich kids, I believe they've created a special unit to deal with them. (Joke!! I think)
The have a great publications division too, Irma Boom designed their most recent publication, and a fab bookshop is downstairs. Need I say more...?
Otherwise the course at Cambridge is well respected, and possibly, but less certainly, the RCA in London.