I like some of the old Airfix stuff, but the real nostalgia for me is Matchbox kits, they really bring back some memories for me.
They were real easy, no real hang ups in construction and they were great for the shorter attention spans of younger modelers, plus they were also cheap enough that you didn't have a fit if you screwed up on them.
Fond memory makers:
Matchbox 1/72 F-101 Voodoo
Matchbox 1/72 EE Lightning series
Matchbox 1/72 EE Canberra PR.9
Matchbox 1/72 Bae Hawk 200
Airfix 1/72 EE Lightning F.3
Airfix 1/72 Sepecat Jaguar
Airfix 1/72 Bristol Blenhiem IV
Heller 1/72 SAAB 29 Tunnan
Heller 1/72 Gloster Javelin T.3
A real happy memory maker for me was Revell/Crown 1/144 B-52 Stratofortress. Real simple, real fun and about what you expected for the money.
About old kits not supposed to be built: thats like wine collectors saying not to drink such and such a vintage because of its rarity. You only live once and the guy who stomped those grapes wanted you to drink it and savour it not put it on display like some collector's plate or porcelain doll. The model manufacturers
don't do what they do for you to stick the kit in the back of some closet and not do so much as remove the shrink wrap.
Build the kit, drink the wine (yes, in that order. I've seen it the other way around and its just bad news)
Live, live, live!!!