Stiffest Flat Pedal Shoe?

Trials boots are pretty soft. I haven’t found anything lighter/more comfy that work on pegs.

I haven't ridden every 5.10 shoe, but out of the few that I've ridden the Impact low definitely has the stiffest sole (compared to Impact VXi, Freerider, and Freerider Pro)

This just dawned on me, what about getting a clip-in style shoe, one with a sticky sole, and not taking off the cleat cover?

Something like the Giro Chamber or the Five Ten Hellcat...

I had an old (maybe 2009?) pair of 5.10 Impact Lows, and last year replaced them with a new pair of the latest Impact Pro. The new Impact Pro sole is way stiffer in comparison, it has little flex on the pedals.

What about the impact high tops? Wouldn't they be stiffer?

The first specialized 2f0 shoes are really stiff. Best pedaling flat shoe I've tired. At first I hatted them because they were so stiff, then they broke in and provided a bit more feel while still being stiff enough to pedal quite a bit better than any 5.10 shoes I've owned. As you would expect, traction isn't as good as 5.10. I had mine resoled with 5.10 "aqua stealth". Same as s1 just a little softer. Killer combo. Traction was bonkers, and pedal power was great. I would do that combo again but it's becoming harder to find resole kits, and first gen 2f0's. The second gen 2f0 is supposed to be flexier because some didn't like how stiff the first model was.