Completion rates I posted in another thread, with D:OS2 up to date (8.5%->9.8% since January):
- Mass Effect 2: 56%
- South Park: Stick of Truth: 47%
- Mass Effect 3: 42%
- Deus Ex: Human Revolution: 37.2%
- Dragon Age: Origins: 36%
- Dark Souls 2: 33%
- Deus Ex: Mankind Divided: 32.4%
- Skyrim: 32%
- The Witcher 3: 26.5%
- Fallout 4: 25.2%
- The Witcher 2: 18.9%
- Invisible, Inc: 18.9%
- The Banner Saga: 16.4%
- Tides of Numenera: 14.4%
- Dragon’s Dogma: 12.1% (2.8% twice)
- Legend of Grimrock: 10.3%
- Pillars of Eternity: 10.2%
- Divinity: Original Sin 2: 9.8%
- Legend of Grimrock 2: 8.3%
- Blackguards: 7.6%
- Divinity: Original Sin: 6.6%
- Expeditions: Conquistador: 5.1%
- Darkest Dungeon: 2.1%
Some games have an achievement for each different ending, so I did some estimates ranging from 1-30% of players doing more than one ending, capped at pre-ending achievements if available:
FNV: 17-24% (19-27% if you only include players who got past Doc Mitchell)
Dark Souls 3: 30-37%
Tyranny: 20-27%
Wasteland 2: 5-6%
Shadowrun Dragonfall: 11-15%
Shadowrun Hong Kong: 25-33%
[Note: BioWare released their own numbers and included consoles]