Depends on what you're up to. My current favorite off-road bike is an old steel 26er I modded out, in fact I like it so much I sold my 29er, now granted that was mainly to free up room and money for more vintage bikes ... but still, for the particular off-road trails I'm riding these days (short, tight and punchy) I kinda like having a steel 26er. It's fun.
Thing is, I would absolutely not want to ride those trails on an early 90's rigid race bike, feel like I'm taking my life in my hands -- on not particularly difficult trails, the blues are VERY tricky on those old frames. To me anyway, and it's probably just me, but still, I found myself on a late 90's frame, one that came with a sus fork to begin with. I was able to squeeze an old Fox 80mm fork in there without screwing up the handling. Decent wide range clutched 1x, saint shifter, XTR brakes, MX handlebars raceface cranks. About the most fun you can have with your clothes on.
I'm personally on a Trek 930 which is pretty decent steel all things considered. Do it all over again, I mighta based that build on a kona lava dome or better from the same era, although those TIG welded 900 series Treks sure give a lot of bang for the buck
I wouldn't much want to dip into the 2000's models, not a huge fan of that era, exception being the on-one steel 26er frames, yall remember those? I had an on-one 456 bitd, that frame was schweet! "We thank you please" lol. Not a fan of aluminum, which feels harsh to me and is prone to metal fatigue.
Wouldn't want to go much older either; among other things, 80's bikes tend to be heavier and I'm not a huge fan of lugged frames off road (potential point of failure). With exceptions; my '87 ritchey would make for a fine off-road bike but believe me, that thing isn't going anywhere near a molecule of dirt (plus I've gotten myself used to running a decent sus fork and it's darn hard to find em with 1" steerer tubes)
