Genuinely good movies that have went under the radar?

To take another angle, I'd suggest some forgotten films from mainstream auteurs; particularly those directors with larger than average CVs, where films in their back catalogue have become lost among the more famous / better remembered features. So for example:

Empire of the Sun, Steven Spielberg; In the mid-80s, Spielberg made his first stab at directing less crowdpleasing, blockbuster entertainment - this and 'The Colour Purple' stand out between the ETs, Indiana Jones & Jurassic Parks. With the lead played by a young Christian Bale, it's a gritty POW story in WW2 Singapore, yet being early Spielberg still has that starry-eyed quality of his other early work.

THX-1138, George Lucas; it's crazy to think that before Star Wars, Lucas was known as a plucky, creative indie director. THX-1138 is a dystopian tale that's very typical of the 1970s, bursting with pessimistic energy.

Paths of Glory, Stanley Kubrick; A fantastic anti-war, courtroom drama from Kubrick. A lot more 'standard' than his more famous, more genre specific work such as Shining or 2001, but still a gripping watch. Brilliant central performance from Kirk Douglas, and film itself opens with a spectacular battle scene.

The King of Comedy, Martin Scorsese; a forgotten film that loses out against the likes of Raging Bull or Tax Driver. Jerry Lewis in a rare dramatic role, while DeNiro returns to the 'dangerous psycho' well of acting.

Miller's Crossing, Coen Brothers; When the 'best of' Coen lists pop up, Fargo, No Country for Old Men, Big Lebowski get the plaudits - but for me this is up there with Coen's best work, with a career best performance from Gabriel Byrne. The soundtrack remains quite beautiful.

Insomnia, Christopher Nolan; compared with the grandeur and epic scale of Nolan's later movies, this dark psychological thriller feels positively quaint in comparison. Noteworthy too for occurring during a phase of Robin Williams' career when he was taking on dramatic roles (which reminds me, One Hour Photo is a genuinely forgotten gem from that phase, Williams playing a disturbed shop clerk stalking a normal family).