Back when DLCs were called Add-Ons/Mission Packs on PC, Commandos: Behind Enemy Lines (1998) had an excellent mission pack called Beyond the Call of Duty.
Around the same time Unreal (1998) had an add-on called Return to NaPali and almost everyone was disappointed in it.
Deus Ex (2000) - a game that pushes the boundaries of what the video game medium is capable of to a degree unmatched to this very day.