There are probably a zillion lost films that are better movies than LONDON AFTER MIDNIGHT.  I'd dearly love to see DER JANUSKOPF, A BLLIND BARGAIN, LIFE WITHOUT SOUL, IL MOSTRO DI FRANKENSTEIN, DRAKULA HALALA, THE HAUNTED HOUSE, and on and on. 
And, apart from horror films, another of my obsessions is to see every movie ever nominated for an Academy Award, so THE PATRIOT, WAY OF THE FLESH and about a dozen others are high on my list. 

But the question posed is "Is LONDON AFTER MIDNIGHT the Most Sought Lost Film?"  I think no matter how much better other films might be, and no matter how much it might aggravate a lotta folks -- particularly silent film buffs -- the answer to the question at the top of this topic is undeniably "Yes."