38 Special-
Easy and cheap to reload(I can reload cheaper than current "street prices" for 22LR), accurate, enough recoil to know you're shooting a gun but not so much as to be uncomfortable, and chambered in a huge variety of guns.
If I had Dillon, it would spend probably 90% of its time loading 38 special. Even on a single stage, I often reload it in 1000 round "batches."
I'll also add that with the current shortage of 22LR, when I take a new shooter to the range, I often start them off shooting my 357 lever gun with some light loaded 38 specials. In such a configuration, the recoil is only marginally more than a 22LR(I've yet to have anyone find it uncomfortable) but has the advantage of allowing the shooter to more easily see their holes in the target.