Depends.......I USED to have 12, 20 and 28 gauge guns, reloaders, components, etc. Once I saw how easy it was to download my 12 and 20 to 28 levels, I had no need for that third gauge. 95% or my shooting is at clay targets, so a 12 is my primary choice. I also have 20 guages I use for occasional targets and mostly bird hunting.
Skeet is a 4 gauge game - 12, 20, 28 and 410 - if you are going that route. Some folks only shoot in the 12 gauge event, but they might shoot a 12, 20, or 28 doing so.
What's going to work for you? Are you mostly a hunter looking to stay sharp in the off season shooting skeet? Are you interested in trap or sporting clays? Lots of things to consider.
If your primary objective in life is upland birds like dove and quail, get your nice 20 or 28 and shoot skeet with it to stay sharp.
If your primary objective is skeet competition and hunting in the fall, get the 12, a set of subgauge tubes in the other gauges and go have fun