Use of Weapons and Consider Phlebas are the worst of the series IMO. I powered through Consider Phloebus just because I knew people loved the series, but there's really no reason to start there
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.