Sam Fuller is kind of frustrating - obviously a super driven guy, almost to the point of crazy, a real character, he churned out his movies whether he had money to work with or had a shoe-string budget, and the New Wavers loved him to death. But: he's made far less masterpieces than the Criterion collection/generation will have you believe, in my opinion.
I agree that Shock Corridor and The Naked Kiss are so-so (Kiss is better than Corridor). I'm not that familiar with earliest stuff (cinephiles go nuts over The Steel Helmet, Park Row and I Shot Jesse James), but for me the Essential Fullers are: Pickup on South Street, House of Bamboo, The Big Red One, White Dog, Forty Guns. One noir, one post-war noir, one WWII action, one contemporary thriller, one Western. Nice spread.