-Big pads helps
- vents with unimpeded flow to exhaust ports are cooler
- ventilation just above the eyebrows is good for evaporation/cooling and stops sweat pouring on your sunglasses

For example
Kask protone
Specialized airnet, echelon

Oppositely, some helmets have huge vents but have the brow/forehead unventilated and will still be hot. For example my friend has the giro aether or cinder which looks well ventilated but has the band blocking the forrhead

The catlike mixino seems to be shaped to catch air like a bonnet scoop which is good for cooling at lower speeds like hill climbing

Of all the helmets I've used, the Endura Airshell has been the best ventilated by far.

For regular road rides now I'm using a Specialized Prevail, which is pretty cool, though I notice my brow gets a bit warmer (the forehead vent is OK but the result is the main vents don't come down quite S far so there's a 'band' on my forehead that doesn't get as much air, which I nitice a bit. Not bad at all and a very comfy lid. But the Airshell was more breezy.

I can't speak to all the helmets on the market, but I'm a very heavy sweater and I live in the super humid south. My giro synthe has been absolutely amazing in terms of keeping my head cool

POC Ventral Air. Much air flow - it is aptly named.

This is very true. I have a Kask Utopia, which is a full aero road helmet but still ventilates better than most other helmets I own. The great ventilation is largely because the Utopia has large air gaps around your forehead. The pads in the Uptopia and oriented front to back rather than side to side in order to create these gaps. The upside is air howls through there at speed, it's remarkably cooling. The downside is that there's a lot of pressure on two thin pads on your forehead.

Other recommendations:
- Kask Valegro
- pretty much any POC helmet, the Omne and Ventral both vent particularly well

I'll second the Giro Synthe. I live in NE Florida and tried to suffer through hot rides with a Smith Ignite (the aero one with the three huge vents in the forehead area). I thought I'd be fine with it, but the Koroyd stuff they use seems to block all of the wind and I was sweating quarts down into my sunglasses.

I switched to the Synthe and have been completely happy with it. My head stays cool even with a cap underneath.

 
If you're willing to set aside not only aerodynamics but protection...we used to rock these spectacularly ventilated units:

Looks like playmobil lol

Check out the Louis Garneau Course. Not only covered in lots of large vents, but the cutouts are pointed forward instead of perpendicular to the helmet surface, like some of the other helmets mentioned. The Course ends up catching more air and it claims to be more aero since air can both enter and exit the helmet more easily.

+1 on the LG course. Of my half a dozen helmets, this is the coolest for road biking. Even cooler than the synthe for me.

For mtb I wear a lazer helium - seems to channel sweat better and allow better ventilation at low speed.