My top five favorite LEGO helmets are:
#1: The pointy visor helm... When those came out, I literally was dancing with joy (especially since you could get them relatively cheaply in the #6009-1 Black Knight
set... My biggest single regret of giving away almost all of my sets during my first Dark Age was losing those helmets (and when they included it in the new Kingdoms Advent Calender, I of course had to buy one).
#2: The Morion from the Imperial Armada... I've long been a fan of 15th-17th century history (an area not normally covered by LEGO), and really like how the Morion looks.
#3: The new Great Helm (although, personally, I like the Brickforge Great helms better than the official LEGO ones), because it is the single most awesome helmet from the medieval period, IMO.
#4: The Kettle Helm, because it is not only historically accurate, but it also can be used for more than just Castle stuff (I've seen it used quite a bit in Steampunk MOCs)
#5: The Dragon Helm: It's just perfect for some evil Nazgul types.