Sacrilege! Taste is subjective, but Use of Weapons, imo, is Banks best work. I personally consider it one of the best SciFi, period. It's been years I last read it, but that ending still gives me shivers whenever I think of it

Use of Weapons is generally considered one of the best, but it has a complex narrative structure that makes it a harder read, and it's probably not a good place to start.

Personally, I think Use of Weapons is by far the best of the Culture series - although I admit it took a few readings for me to get to that view...

He does a really good job exploring the theme "what is a weapon?"

Yes! And also, "why is a weapon?", "how is a weapon?" and "what isn't a weapon?"

Really, Culture is a great setting for discussing a lot of interesting philosophical questions and topics.