My original suggestion for a game with a high ratio achievement was Destiny:
It has an achievement called "Flawless Raider" which was a 22.17 when I posted, but rapidly dropped after
The Flawless Raider achievement in Destiny worth 257 pointsComplete a Raid without anyone in your fireteam dying. QuickMythril has suggested the following two achievements which both look pretty crazy:
The solution for the above achievement says "This might be the most difficult achievement on the Xbox 360"!Sliktor also chimed in with this one:
The Hit Maker achievement in Project Spark worth 69 pointsReceive 50,000 downloads on all your worlds or play 14 hours of Community Games. But I think that
BastiXS96 has taken the crown with this one in Pinball FX2:
The Perfectionist achievement in Pinball FX2 worth 7658 pointsComplete all the main modes in a single game on the Sorcerer's Lair table! (Single player only)