[{_id:1,"changelog":[{"Title":"Foo","Director":"Bar","Genre":"Action","rating":3.45},{"Title":"Foo","Director":"Bar","Genre":"Action","rating":4.96},{"Title":"Foo","Director":"Bar","Genre":"Action","rating":4.99}]},{_id:2,"changelog":[{"Title":"Foo","Director":"Bar","Genre":"Action","rating":1.45},{"Title":"Foo","Director":"Bar","Genre":"Action","rating":1.96},{"Title":"Foo","Director":"Bar","Genre":"Action","rating":2.99}]},{_id:3,"changelog":[{"Title":"Foo","Director":"Bar","Genre":"Action","rating":1.45}]},{_id:4}]

db.collection.aggregate([{$addFields:{changePercent:{$subtract:[{$arrayElemAt:["$changelog.rating",-1]},{$arrayElemAt:["$changelog.rating",-2]}]}}},{$sort:{changePercent:-1}},{$project:{changePercent:0}}])