Hello friends!
There are few top-5 and top-10 LH topics, i want to suggest one more. Let's discuss lighthouses, which are most hard to get. Located in very wild places, and couldn't be easily reached.
I can suggest one mine: Yuzhnyy Goglandskiy:

It's located in one of the island in the Gulf of Finland, near Russian to Finland border. Population of the island is less than 50 people, mostly coastguard and LH keepers. LH was built in 1905. Currently there are 2 ways to get the island - by helicopter (1-1.5 hour flight from Saint-Petersburg) or by sea (may be one day from nearest port). Transportation is available in best case maybe one time per month.

OK now your turn :)