This Medaka Box page is a good guide about the main Jump manga before 2000
In chronological order:
Cobra (Cobra)
Kinnikuman (Kinnikuman)
Captain Tsubasa (Tsubasa Oozora)
Fist of North Star (Kenshiro)
Dragon Ball (Son Goku)
City Hunter (Ryou Saeba)
Saint Seiya (Pegasus Seiya)
JoJo (Jotaro Kujo)
Sakigake Otokojuku (Momotarou Tsurugi)
Hana no Keiji (Maeda Keiji) (this must be the most unknown manga from the list. 1990 historical manga that achieved 20 million of sales, by Tetsuo Hara, the same artist of FotNS)
Rokudenashi Blues (Taison Maeda)
Video Girl Ai (Yota Moteuchi)
Dragon Quest Dai no Daiboken (Dai)
Slam Dunk (Sakuragi Hanamichi)
Yu Yu Hakusho (Yuusuke Urameshi)
Kenshin (Kenshin Himura)
Sexy Commando Gaiden (Masaru Hananakajima)
Hoshin Engi (Taikobou)
Yu-gi-oh (Yugi Muutou)
Hikaru no Go (Hikaru Shindo)
Prince of Tennis (Ryoma Echizen)
---
The oldest manga after the Yu-gi-oh/HnG/PoT trio were already serializing manga: One Piece, HxH and Naruto, so I guess that page avoided current works in serialization in that time. Then, the oldest manga already ended was Eyeshield21. Shaman King also isn't in the list, I think it wasn't still banned when that page happened. That would also explain the lack of Kochikame. I also think the list is 1 manga per author, since I's, Rookies, Dr.Slump, and Ring ni Kakero aren't in the list.
Elaborating further the list, and seeing how all the titles practically have a minimum bare of 20 million copies, I'd add:
Kochikame (Ryotsu Kankichi)
Ring ni Kakero (Ryuji Takane)
Dr.Slump (Arale Norimaki)
-----
One Piece (Monkey D.Luffy)
HunterxHunter (Gon Freecs)
Naruto (Uzumaki Naruto)
Bleach (Kurosaki Ichigo)
Eyeshield21 (Sena Kobayakawa)
Death Note (Light Yagami)
Gintama (Sakata Gintoki)
D.Gray-man (Allen Walker)
Katekyo Hitman Reborn (Sawada Tsunayoshi)
To-Love-Ru (Rito Yuki/
Lala Satalin Deviluke) (I'd put Lala, but then Video Girl Ai had Yota in that pick over Ai)
Toriko (Toriko)
Kuroko no Basket (Kuroko Tetsuya)
Nisekoi (Ichijo Raku/Chitoge Kirisaki) (I was doubtful in adding it since I think Nisekoi hasn't even reached the 15 million copies, not sure)
Haikyu (Hinata Shoyo)
Assassination Classroom (Koro-sensei/Nagisa Shiota)
Shokugeki no Soma (Soma Yukihira)
My Hero Academia (Izuku Midoriya)
Kimetsu no Yaiba (Tanjirou Kamado)
The Promised Neverland (Emma)
Unfortunately, Black Clover is forever stuck in 10m hell. And it seems Jujutsu Kaisen has more chances to join the list than Dr.Stone in the future. Having my doubts with adding or not I's and Rookies
PD: Is funny and sad at the same time how Whistle, despite achieving almost 20 million copies, seems it doesn't exist for Shueisha