I'm just wondering what you consider to be the best games published by SSI back in the day that do not use the D&D/AD&D license. (In particular, I'm looking at games like Demon's Winter and Phantasie, not games like Pool of Radiance or Eye of the Beholder.)

So, any thoughts?

Gemstone Warrior and Gemstone Healer, basically Diablo before Diablo.
Rings of Zilfin, an adventure told through minigames, very unique and fun back then.
Questron 1 and 2, Ultima clones, I never played Ultima but I liked these.
Panzer General, great strategy game although RNG shenanigans turns it more in wheel of fortune sort of game.
World of Aden series, played them but never finished them.

Honorable mentions, Silent Hunter series, Su-27 Flanker, had some fun with these but not really my kind of games.

The list is not too big. I'd say Wizard's Crown and Eternal Dagger were the most solid examples of detailed tactical combat, in the vein of their wargames, bundled in an old school, rpg. They were also the proto basis of what later was the gold box combat system in Pool of Radiance, polishing, revising things Warlords were made by SSG, not SSI

For me:

"Field of Fire" (1984) and "Battle of Antietam" (1985).

SSI was such a wnoderfull publisher. I will just add games that were not mentioned jet:

5 Star general series (Allied Generral, Fantasy General, Pacific General Star General and - already mentioned - Panzer General)
First Warhammer Games:
Final Liberation
Chaos Gate
Rites of War

Buck Rogers: Countdown to Doomsday
Buck Rogers: Matrix Cubed

Both games use the same gold box engine that was used for a lot of the dos-era D&D license games. They also both languish in licensing limbo. Someone owns the rights to these games and got them pulled off abandonware sites, but nobody has brought them back into print.

Community Wishlist links:
https://www.gog.com/wishlist/games/buck_rogers_countdown_to_doomsday
https://www.gog.com/wishlist/games/buck_rogers_matrix_cubed

The Steel Panthers series
I remember having a ball with the demos of these tactical games. I learned about different military equipment. There was a morale setting you could change on each side to make it a cakewalk or a hard fight. Also had bombing runs and off-map artillery support.

From what I can tell about your tastes (and especially given the examples you used), you probably should've specified "SSI RPGs", given that, in their heyday, they were perhaps better known for real-world-military/war-focused strategy and simulation games than for RPGs (their D&D titles being a partial exception). ;)

I second Steel Panthers. The one I played in the day was III, but the first two are also fine.

If you're looking for blobbers, maybe take a look at Thunderscape. I can't attest to its quality as I have yet to boot it up, but I did buy it a while back as a curiosity purchase.

Also there's Veil of Darkness which I have played and is kinda fun. It's not on GOG, but you can play it on Internet Archive last I checked. It's more of an isometric adventure game with some light RPG elements. It also had an unrelated predecessor in the form of The Summoning and DarkSpyre. There are other Event Horizon RPGs, but they're not associated with SSI.

I've always had a soft spot for Roadwar 2000 and its sequel, Roadwar Europa. Post-apocalyptic street gang management/strategy games.

lol close been like 26 years ... :/

Indeed, :) both companies more or less comemporary, similar name and they dedicated to the Wrgaming tititles basically, SSI diversified though.

Warlords and Carriers at War were self-published by SSG for sure, though most SSG games after that (like their Campaign series) were published by SSI. SSG were just a tiny Australian developer, they lacked global publishing expertise- hence their later alliance with SSI for publishing.

Anyway, SSI were the greatest publisher and developer in video gaming history IMO. To this day the Great Naval Battles series is still unmatched- true real time tactical games from the early 90's where you could play the entire naval theatre of your choice during WW2 in genuine real time, one second equals one second. Good luck with that, I'll see you in 5 years. Or you could do what normal people do and speed up time when nothing is happening.

And then there was Soldiers at War, Necrodome, Fighting Steel, Destroyer Commander, Su27 Flanker 2.5, Panzer Commander, they actually put out very few games I didn't like. If only SSI could have just hung on long enough to usher in the digital age, then they may have survived as a niche publisher/store like what Matrix Games is today.