As with "most" things in 3-gun, the appropriate shot size is most often dictated by target type and personal preference. I run Remington Heavy Dove 1 1/8oz, 7.5 shot for probably 75% of targets I shoot in competition.
For close-in clays(10 yards or less), I use either 8 or 8.5 Fiocchi Spreaders.
For Heavy Steel, steel that's "sticky" due to mud on the base, or for those times where two targets are lined up and I want to use one shot to knock both down, I use high-brass pheasant loads, usually 1 1/4 oz of either 4 or 6 shot.
For the few matches that allow it, I sometimes use #4 buck for extra heavy steel or if I'm shooting a very long distance at steel, typically from an advantagetous "gamer" position.
If shooting an MGM Spinner, I'd use Federal Flight Control Buck (assuming the match allows the use of Buck). If the flight conrol buck wasn't an option, I'd go with the high-brass pheasant loads.
For Slugs, I use Fiocchi 1oz reduced recoil slugs because my M2 shoots them accurately and reliably, but this can change from gun to gun. My back-up M2 doesn't shoot the Fiocchi slugs as accurately as my primary gun. Some prefer S&B, Remington Reduced recoil slugs, or other types.
Hope this helps.