I think there is a wide consensus that Consider Phlebas, though first, is not the ideal starting point for Banks' Culture series. The plot is meandering, and compared to later books, it is clear that Banks had not yet quite found the form that would be so impressive later on.
I usually recommend The Player of Games, or alternatively, depending on the reader, Use of Weapons as starting points.
It is however a good idea, having read other Culture books, to read Consider Phlebas before beginning Look to Windward, as there is a connection between the two.
I wouldn't recommend starting with Use of Weapons - it is probably the best Culture novel but it took me a few readings to really appreciate it.
Edit: As far as I recall, Banks originally created the Culture as a background for the protagonist of UoW - the ultimate warrior:
"The method was that taking and bending of materials and people to one purpose, the outlook that everything could be used in the fight; that nothing could be excluded, that everything was a weapon, and the ability to handle those weapons, to find them and choose which one to aim and fire; that talent, that ability, that use of weapons."
Player of Games was the first Culture novel I read and I enjoyed it a lot. I still sometimes go back and reread it and have found new perspectives each time. Would definitely recommend.
I enjoyed Player of Games and it was a good second book for me after Consider Phlebas. I’ve heard a lot of good things about Use of Weapons, but I haven’t been able to get into it after starting and restarting a couple times.
It is however a good idea, having read other Culture books, to read Consider Phlebas before beginning Look to Windward, as there is a connection between the two.