There's probably better, more trafficked places to post this.
The Ops-Core FAST is THE standard modern helmet and is used by basically every major US military SOF unit. USASOC (Rangers, SF, etc...), AFSOC, MARSOC, NSW, some USCG DSF, as well as many major Fed LE and local and state LE agencies. It's level IIIA like the current issue ACHs but uses different construction and materials.
There is also a FAST "Maritime" version that's even lighter (and ~50% more expensive) , but has reduced ballistic capabilities (Level II) and was developed as more of a "ballistic bump" helmet to provide blast/frag protection to units who had traditionally opted for lighter nonballisitic helmets like Pro-Tecs for some tasks. This seems to be the choice of some JSOC SMUs (the same kind of guys who forego boots for Vans DEFCON skate-shoes).
This leads to the Airframe. The Airframe is a Level II helmet with coverage similar to a TC-2002 mid-cut helmet, and has a small joint that allows Airflow through the helmet.iht we Seems they are used in the same roles as the Martime, and are popular because of their light weight. However, that light weight comes at the cost of ballistic protection; which like the Maritime is blast/frag protection where skate helmets were previously used.
Crye has another version of the Airframe called the ATX. This IS a IIIA ballistic helmet which they won't publish a weight for, which leads concern for a significantly increased weight.
The Team Wendy EXFIL is a IIIA helmet and seems to be the lightest out there. I'm not aware of any major organizational use, but Pat Rogers and some other trainers have been pimping the crap out of them.
The revision Viper line of helmets is basically revision's version of the military issue ACH. IIIA, available in full, mid, or high-cuts. Slightly heavier, but substantially cheaper (~1/2 the cost of the Cryes, Ops-Core, or Team Wendys). Don't discount the military issue helmets. The high and mid-cuts work great with comms and the full cuts provide a ton of coverage. They're still very relevant choices.
Personally, I would only look at IIIA helmets considering the possibility of being shot at as an LEO. I would also see what is available via the 1033 program, you might be able to get a whole bunch of free military issue helmets.