ayy, old wads are my forte! this thread is perfect for me :)
these are all fairly well-known if you're big into 90s stuff, but hopefully you haven't played at least a few of them
KILL! Zone from 1997 is a fun 7-map episode that has a lot of fairly large, somewhat exploratory maps. the texture alignment can be somewhat awful at times, but it kinda gives it a certain charm.
Dark Covenant, made in 1996, is a wad with 12 visually impressive (for the time) and action-packed maps filled with monsters, and while they are a bit too linear at times, it's generally regarded as a pretty fantastic set of maps for its time. fun fact: map07 (i think?) is referenced in scythe's map30 :)
Insertion is an 11-map wad that features some of the same people who worked on the memento mori series. while i personally don't think it's as good as memento mori 2, it's still typically regarded as somewhat of an extension of the series, fitting into the same slot as requiem, which came out that same year.
Realm of Shades is a 7-map 1998 wad that, while tame compared to a lot of the stuff released nowadays, is still shockingly difficult for its time. i was finding myself regularly scraping by with barely any health for most of my playthrough. the ending of map04 is especially brutal - goddamn, that one is a motherfucker. however, it's still hard in a fun way as opposed to a frustrating way.
Operation BIOWAR is a short, easy 18-map wad that's kinda focused more on adventure than anything. there's a lot of neat visuals, doomcute, and it honestly feels like a better looking and more consistently good icarus or tnt.
Mars War is the perfect wad if you're looking for the most 90s thing possible. released in 1998 and updated in 2009, this full 32-map megawad is full of surreal, quirky, and oftentimes terrible looking maps that are nevertheless incredibly charming and have some decent gameplay to boot. it's a cult classic for a reason :p
Phobos - Relive the Nightmare is a 9-map 1996 wad made by the late shamus young. it's a fun e1-style episode that feel as if the original e1 maps were designed to also accommodate doom 2 monsters. it's good fun.
i could probably list off more, but, eh...i gotta go to bed