Look to Windward.

It’s a very human story, and the technobabble intervenes very little in it. It also has the most connection (of the Culture novels) to our world, being largely a meditation on the first Gulf War. This makes it the most approachable, in my opinion.

It is technically a sequel to Consider Phlebas but the connection is almost nonexistent, no prior knowledge of the Culture-Idiran war required.