If I had read Consider Phlebas first I probably would not have read the rest of the Culture series, I really strongly dislike it. After reading other Culture books, it does have some interesting stuff about the Idiran war, but it's so atypical, and the main character so fundamentally unlikable, that it makes the worst possible introduction to the series.
Excession is my favourite but it's not a good one to start with in my opinion.
Consider phlebas is a pretty straightforward one. It's got a normal timeline and does a good job of introducing the culture and their history.
Don't sleep on his non culture SciFi story the algebraist either, it's excellent.
If I had read Consider Phlebas first I probably would not have read the rest of the Culture series, I really strongly dislike it. After reading other Culture books, it does have some interesting stuff about the Idiran war, but it's so atypical, and the main character so fundamentally unlikable, that it makes the worst possible introduction to the series.