I'm afraid it wasn't an American comic. What it was was an Australian Phantom (Frew), that I saw in a newsagents window in a Fife town around mid '50's - the family was on holiday. I never actually got the comic as it was much dearer than a British title and my mother thought it was "too old" for me, but the image haunted me and I was hooked. Thing is, even now when I have a big collection of Frew Phantom books, I don't know which one it was I saw all those years ago.
The other comic that had a huge impact was Showcase 23, with that amazing "Invisible Destroyer" cover. For a wee boy of around 10, it was a revelation. This was just on the edge of American comics starting to be imported and distributed in newsagents here. Not quite sure but I think the price, which was stamped on the cover, was 9d - dearer than the standard British comic of the time. But this was all in colour. That comic started my lifelong addiction to comics. And coupled with my memory of The Phantom, confirmed my love of superheroes and Masked Mystery Men.