I'd start with Consider Phlebas. It's the most space-opeartic work of the series and although it's a somewhat early description of the culture, it pulls off a unique trick of describing the main protagonists of the series from the point of view of a sworn enemy (who is also a very likeable hero).
The other books suggested here, Player of Games and Excession are also quite good, but nothing beats Phlebas for sheer energy.
Well, opinion anyway!
P.S. The State of the Art, a collection of short stories with a few Culture ones (including the namesake) is also a good introduction to some of the themes and ideas in the later novels.
