My current 50 most rare trophies are from 11 games (12 when you take the same game on a different platform as a different game), which are, ordered by the rarest trophy first, the following games:
The Crew (1) - 10
ModNation Racers - 5
Grand Theft Auto IV (The Ballad of Gay Tony) - 2
Grand Theft Auto V (PS3 and PS4) - 9
When Vikings Attack! - 2
ModNation Racers: Road Trip - 6
Driver: San Francisco - 5
Pumped BMX + - 8
Need for Speed: The Run - 1
LittleBigPlanet Vita - 1
Grid 2 - 1
This ranges from 1.06 for King of the Road (The Crew platinum) to 3.89 for Every Trophy Counts (Grid 2 platinum).