I can only speak for the server "Siren."
Here's a top list for Siren from approximately 2006-2014.
Best Leader: The top spot goes to "KUZ" or "Kuztastic."
From 2006 to 2014, Kuz, Sacia, and the other leaders kept the Linkshell raiding three times a week without interruption, showcasing remarkable discipline and stability.
I've been in several end-game Linkshells simultaneously and post-2014, on private servers, FFXIV, and WoW, I've never met anyone with Kuz's leadership abilities. In fact, his group was still raiding until about a year ago.
From here on, I'll only select players based on their performance in "Parse" and their ability to adapt to situations.
Best PLD:
Peacepeeker from "Siren" and Xpeacekeeper from "Asura" is the best tank I've known, no words needed.
Best BRD:
Bruno from Siren. He even has a guide on the forum for DD-BRD. I saw him solo Krezeut at level 75 with his BRD without dual-boxing, and the damage his BRD dealt was absurdly high.
Best SAM:
Peacekeeper. To this day, the guy is unstoppable in the parse, with only a few able to compete with him.
Best DRK and PUP:
Marth from Siren. He and PK were undoubtedly the two best DDs I faced in the parse for about six to seven years. It was a blessing and a luxury to have them in your group.
Best WHM:
Shinshi from Siren. Having her in your group was a luxury. Xanababy and Elysian were also amazing.
Best THF:
Melphina from Fenrir. Since she posted on Allakhazam, she was a top player in wisdom, knowledge, and the parse.
Best WAR:
Saevel, without a doubt. I saw him play a few times, and his Linkshell helped me get items. The guy is a monster in the parse.
Best DRG:
IHM. His DRG was unbeatable in the parse. He would silence critics when people had the mindset of LOLDRG. With this guy, we could clear all the chambers of Legion in his Linkshell "chocobogordo."
Best MNK:
Vegetto. He used to post a lot on this site years ago.
Best BLM:
This is a technical tie between Rinzhu and Darkimp. Both were god-tier BLMs.
Best NIN:
Ruamoko. He helped me several times when I returned to the game in 2020, and his NIN was impressive.