It's always interesting to me, who just started delving into Bank's Culture/SF novels last year, to see how long time readers rank the books.
So far, I would recommend any of them to the thoughtful reader of SF. And unlike some others, I don't have a clear preference for Culture vs non-culture novels. In fact, the first Banks I ever read 10 years ago was The Bridge, which is not that far from the most recently read, Transition.
I'm currently re-reading Feersum Endjinn, which I like a lot. Amog the "M" novels still to be read: Look To Windward, Excession, Inversions and Against A Dark Background. I enjoyed the Iain Banks novels The Crow Road, Whit, Espedair Street and, of course, The Bridge, the best non M book IMO. Don't know quite what I think about The Wasp Factory, a sublimely nasty, if brilliantly written, little tale if ever there was one.
I concur with others that Player Of Games is probably the easiest for the Banks novice to start with, and an excellent introduction to the Culture universe in general.
These are simply my favorites of the moment...like you all, the order of this list changes from day to day, depending on what I've read last, and what I've had for breakfast.
1. Use Of Weapons
2. The Algebraist
3. Feersum Endjinn
4. Transition (It's an M novel in this country)
5. Player Of Games
6. Consider Phlebas
7. Matter