I'm using medium but with the 1.6.x changes; I really feel light should at least be bumped up to 2/4 of the protection of heavy rather than just 1/4 and agree with other posts on penetration being a percentage, etc.
Heavy armor needs more resource management. I would start with reducing the timer on generation from being hit.
I would like to see light be the most damage focused in Magicka. Stamina slightly better damage reduction and most damaged focused in stamina, but that should not be as high as magicka. Then Heavy be tanky and offer a hybrid of the 2 being slightly lower in both magicka and stamina.