In the context of gravel riding (not mountain biking), I am currently enjoying the Soma Supple Vitesse (33mm label, 31mm actual) and the Continental Terra Trail/Speed (40mm label, 39mm actual).
The Supple Vitesse is an open tubular slick with minimal chevron tread and is great on the road and on river trails. I've taken it mountain biking as well and while it handled impressively on climbs, there is zero confidence when descending. If I'm doing purely road and gravel then I don't see the need for more than this.
The Conti Terras (I just happen to have a mismatched set - running Trail up front and Speed in the rear) are also great all-rounders but despite the good road feel on tarmac (Black Chili for the win) the actual speed/rolling efficiency is lacking (Strava doesn't lie). Despite the more pronounced knobby tread, I like these much more than the Conti Cyclocross Speed/Speed Ride (with the file tread) on the road. They handle more challenging terrain reasonably well, better in my opinion that GK SKs and Clement MSOs. They hold up okay on true MTB trails but I'm underbiked for sure.
Currently experimenting with Specialized Pathfinder Pros (40mm label, 38mm actual) on one of my bikes and they seem to be working fine as well. Uninspiring so far, but also unfailing.
So I guess overall, if we're talking pure flat and dry gravel and rough tarmac, I don't see a need to go above a 40mm, or even 35mm fat slick tire. In fact, get the lightest and most supple thing you can find. But as soon as you're doing proper trail descents, even 40mm+ knobbies may feel inadequate. And that's the conundrum that many of us here face.
I'm at a point now where my "gravel" stable is so differentiated: dropbar gravel bike with the Soma Supple Vitesse for all-road/easy gravel, flatbar gravel bike with Conti Terras for gravel/trails, and a hardtail MTB on 2.1"s for purely trail riding.