28s (Conti GP5000s, previously “25mm” Gp4000s which were really about 27-28 wide) on my brevet bike, the widest I can wedge under fenders. I have done a few randonneuring rides on my gravel bike on 38s (Panaracer gravelking slicks) and my performance wasn’t bad—the gravel bike is a little heavier and a little more upright so it’s not apples to apples.
Plus I’ve run the 28mm panaracers on my brevet bike and it was a bit slower than the Contis, although they are definitely faster to change a flat on.