200 Items Or Less: Invasion Of Astro-Monster (1965)
A few thoughts: King Ghidorah is terrorizing an alien planet whose inhabitants enlist the help of Earth to bring Godzilla and Rodan to this planet to fight him. But do they have ulterior motives?!? One of the earliest of the overly ridiculous Godzilla movies, this one features some of the coolest sets of the series since it spends a great deal of its time in space. This is actually the first and only in the series that puts Godzilla in space, which makes it special in its own way. I love the silly alien costumes, the monster fights, and how well the non-monster plot works into the action itself. Usually these movies suffer from poor human plotlines, but this is definitely one of the most well-rounded and thoroughly entertaining. Filled with greed and treachery, the story with the aliens often outshines the monsters themselves, which is pretty exceptional. Maybe this isn't one of the most important or influential monster movies ever made, but for sheer entertainment value, I would place this pretty high up on my list.
Who would I recommend it to? Fans of Godzilla/kaiju movies. This should be a high priority for those planning to look into the genre -- assuming they don't take these things too seriously.