HGU-56
I use a Gentex HGU-56 for last 1000 hours or so, of which 600+ hrs is in MD500 (noisy:-)) and rest in S300 and R44. I had to fit the helmet with zetaliner to make it comfortable for long days (6-8 flight hours). I also use earpads from my Senheiser S1 headset and Alpine MotoSafe eaplugs. I am VERY satisfied with this setup. Does not matter if it is 35deg plus or 15deg minus, it is super comfortable (for me anyway). Had a chance to very briefly try some other helmets but none of them felt so nice.