Put me down for "Green Slime" - it's the SciFi flick that kept me peeking between the fingers longest back when I was a kid...
Steve McQueen's The Blob was a decent sci fi thiller.
but for my money nothing surpassed two movies for bringing out a youthful cheer should they show up on the movie of the week. Runner up was War of the Worlds and top honors went to THEM, giant irradiated bone crunching ANTS. Them was spellbinding. eheh.