If you are referring to vintage figures, nearly every tokusatsu-theme hero has been made (officially or in a bootleg) in an unmaskeable version. For a lot of the Bandai and boot figures, the same head is pretty standard to see. I am not sure how we can narrow it down. It may be a better idea to aim for a particular hero you are looking for. What I think would be really cool is unmaskable or changeable head villains
I also think a Daimajin (being one of my favourite characters) would be so cool with a statue face and 'live' face.Some honourable mention masked toys outside the vinyl world (since you brough up MASK) are the Leia Boussh or Luke Stormtrooper - okay, I know it is a helmet - figure and OG Zartan from the Joe line (this is still one of the coolest mask concepts on a toy I have seen executed).
Spend some time looking through the other vintagevinyl threads, I am sure you will come up with some other excellent examples. I guess I haven't done my fair share of image posting here, but I like and have a few of the less well-known masked heroes, such as Gekko Kamen, Zone Fighter, Rainbowman, etc. You are a bit remiss for not mentioning the obvious Ultra series characters though, tsk tsk, and when you start including nodders, you also have to account for the now much popular Astro-mu line. If you are looking for a cheap start to a masked vintage collection, the Mirrorman and Silver Kamen figures are quite common, and can be had fairly cheaply, especially if you are not fussy over condition.
