I just purchased a Benelli M1 S90 with an 18" barrel and am wondering what choke would be most appropriate for it. It came with the Modified choke installed but I also have the choice of  a Full or IC. I will be shooting 2 3/4" Remington 00 Buck in it.  Has anyone out there been bored or curious enough to have studied choke patterns?

Most combat shotgun makers have settled on Improved Cylinder as the best.

In the old days, straight Cylinder Bore (no choke at all) was the favorite, but Remington did some experimenting and decided that Improved Cylinder gave slightly tighter patterns with buckshot and tighter groups with slugs.

Remington went Improved Cylinder on all their Police guns years ago, and just this year, they seem to have gone with it for all their short shotguns.
Most other makers are following.

Thanks for the info. IC it is!