Lazer Genesis. Best Fitting Helmet EVER!....for me at least

They're fit system is fantastic, there is a dial on top of the helmet that you turn and it tightens a cord all the way around your head, not just at the back. Most people I know who have tried this helmet can't believe how comfy they are.

They're definitely not as popular/common as Bell or Giro so it may be a bit more difficult to find.