All the old Riders, of course.
I haven't seen every last classic Ultraman, but I have seen most, and they're all pretty good. The original, Seven (especially), later Return, Ace & Leo being particular highlights.
The 70's Ishinomori classics (Kikaider & 01, Henshin Ninja Arashi, Robot Detective, Inazuman & Flash, Daitetsujin 17, Zubat.)
Older Sentai- I haven't seen enough in full but they're always interesting. And if nothing else, episode 1 of Goranger is one hell of a first 10 minutes.
The Toei 60's stuff (Giant Robo, Captain Ultra, Akakage)
Supergiant(s) aka Starman- old as it gets, but amazing. Not really a show so much as a series of short films, edited together into four longer films in the US. Watch the English-dubbed versions, Starman's voice is awesome and I like how they keep recycling the "alien council meeting" scene every time. It becomes a sort of strange psuedo-opening, and I missed it watching the original versions!