The Cabinet of Dr. Caligari (1920). Cesare the somnambulist is a human monster, but a monster just the same. This exponent of German expressionism uses painted sets and stagey acting to create a dream-like alternate-universe.
Nosferatu (1922). F.W. Murnau's interpretation of the vampire narrative created what is at least as powerful a vampiric icon as Bela Lugosi's almost a decade later. Consider this the prequel to Salem's Lot.
The Thing From Another World (1951). The first take at what would become John Carpenter's great movie below. James Arness plays the infamous "carrot monster."
The Blob (1958). I must've seen this a dozen times as a kid (or so it seemed), but all I remember is the bulbous, jellied blob pulsing out of the meteorite and also glomming onto a screaming man's arm. Need I remember anything else to love this movie?
War of the Gargantuas (1966). This is how I knew Russ Tamblyn's name when I was a kid. When two giant sasquatch-like brothers destroy each other and Tokyo, you might be moved to tears.
Night of the Living Dead (1968). The original George A. Romero version: best social-commentary zombie flick to date. "If you have a gun, shoot 'em in the head. That's a sure way to kill 'em. If you don't, get yourself a club or a torch. Beat 'em or burn 'em. They go up pretty easy."
Jaws (1975). Best "monster movie?" Just plain best movie.
The Thing (1982). The elusive, tentacled, wildly shape-changing, virus-like alien is a genuinely mysterious creature in a very brooding and atmospheric movie. My favorite of John Carpenter's work.
The Island of Dr. Moreau (1996). A must-see for Marlon Brando's performance alone and, believe me, nothing else. The inspiration for Austin Powers's mini-me character.
Starship Troopers (1997). Who doesn't want to see giant machine-like bugs get blown apart and spew green globs? Like The Thing above and Cloverfield below, this is a combo of sci-fi and monster movie.
Pan's Labyrinth (2006). Egad! I fell asleep watching this, and then woke up, startled, right in the middle of the scene where the naked old goblin-thing-with-eyeballs-in-its-hands slowly chases after a little girl. Apparently Guillermo del Toro's remaking the uber-creepy Don't Be Afraid of the Dark, too.
Cloverfield (2008). See the hipsters get plunged into alien-led Armageddon right in the middle of Manhattan. Until District 9 came along in 2009, this would be the most innovative sci-fi storytelling of the last ten years.
For more reviews, might as well just go to Glenn Danzig's 1983 Flipside movie reviews.