Gaira (the Green Gargantua) from War of the Gargantuas (Toho; 1966). IMHO, he's the scariest giant monster in tokusatsu history (and suitmation actor Haruo Nakajima's favorite role besides Godzilla). I actually had nightmares about this guy!
Also the post-apocalyptic mutants from the now-banned Prophecies of Nostradamus (Toho; 1974). They had me shivering on the spot!
Scary is tough for me because most monsters don't scare me with looks, mostly their abilities (Horrors, Grongi). Any moving dolls I find creepy (Notable one in Dogoo Girl) and probably Chiharu's disfigured form from The Next.