I've been told on more than one occasion that Portland Head Light is the most photographed lighthouse in America. Once someone said the whole world. To be honest, I don't know who is standing around counting all the picture-takers, but I guess we'll have to take 'their' word for it. Whoever they might be.
For my money, though, I'll take the Ram Island Ledge Lighthouse, just north of Portland Head Light on the other side of the harbor channel. It's somehow more stoic, left to its own devices out in the middle of the water. The desolation makes for a more epic feel, as far as I'm concerned.