Highest Difficulty can mean the actual difficulty level or all achievements, all collectibles, full completion, special challenges or multiple playthroughs. Hardcore modes are excluded in most cases.

Battleblock Theatre | Insane (all maps solo)
Borderlands (+DLCs) | Playthrough 2.5
Borderlands: The Pre-Sequel (+DLCs) | Ultimate Vault Hunter
Borderlands 2 (+DLCs) | Ultimate Vault Hunter
Bioshock | Hard
Bioshock 2 | Hard
Bioshock Infinite (+DLCs) | 1999 Mode
Call of Duty: Black Ops II | Veteran
Call of Duty: Modern Warfare 2 | Veteran
Call of Duty: Modern Warfare 3 | Veteran
Dark Souls 2 | All achievements, No death playthrough
Dark Souls 3 | All achievements, No death playthrough
Deep Rock Galactic | Solo Elite Deep Dive
Dishonored (+DLCs) | Very Hard
Elden Ring | All achievements
Gunfire Rebord | Reincarnation 9
Hitman: Absolution | Purist
Left 4 Dead 2 | Expert on all campaigns
Metro 2033 Redux | Ranger Hardcore
Metro Last Light | Ranger Hardcore
Orcs Must Die 3 | Rift Lord
Path of Exile | LVL 90 Harbinger Hardcore + Shaper kill (3.0)
Payday 2 | Death Sentence Solo (all maps)
Portal Stories: Mel | All achievements, Advanced Mode
Resident Evil: Village | Village of Shadows
Returnal | All achievements
Serious Sam 3: BFE | Serious
Sekiro: Shadows Die Twice | All bosses on NG+
Sniper Elite V2 | Sniper Elite
Spec Ops: The Line | Suicide Mission
Strange Brigade | Hard, all achievements
Terraria 1.3 | Expert
Titanfall 2 | Master
The Talos Principle (+DLC) | All puzzles, all endings
The Talos Principle 2 | All puzzles, all secrets
The Witness | All puzzles, secret puzzle maze
Torchlight 2 | Veteran
Wolfenstein: The New Order | Über
Wolfenstein: The Old Blood | Über
Wolfenstein 2: The New Colossus | I am death incarnate!