To save time, and because I don't remember a lot of the servers I worked for, I'm only stating ones that had 100+ players on concurrently.

Velocity Network - I used to be a staff member there, but after the server got bought it died.
Rank: Sr Mod
Concurrent players when it was active: 130
The server went down but has now been revived. https://playvelocity.com/

Impacted Network - I am technically still a developer there, but I am really inactive. Since a lot of players left due to the owner's decisions, I did too.
Rank: Co-Owner / Developer
Concurrent players as of mid-2017: 100-120

Viper Network - This server was super unstable and was the first big network I worked for. The server went down.
Rank: Lead-Developer
Concurrent players as of early 2013: 230