Well 20 years ago i was like 3 but I think Toonami was around a bit longer than that soo....I remember liking watching Naruto when it used to come on Toonami. I remember seeing One Piece there too but it was in the Alabasta arc at the time so I was just kind of lost lol.
Also this may be bad memory but I think that Justice League Unlimited was on toonami as well which was my introduction to the DCAU i think. And maybe Samurai Jack.
I also remember seeing Prince of Tennis, Zatch Bell, and this might be a huge stretch but I think there was a Megaman anime that played on toonami for a bit? idk