Gryffindor: top of the Ferris Wheel, putting the ring in their s/o's food or drink, sky writing, a billboard, hiking to the top of a nearby mountain/hill/nature trail and dropping to one knee at the summit, secretly has the moment recorded by a mutual friend, flash mob, a little drunk but completely sincere, something video game themed, accidentally proposes during an argument, within a week of dating
Ravenclaw: puts the ring inside a hollowed out book, on third year dating anniversary, makes a PowerPoint of all the reasons they should get married with the last slide saying "Marry Me?", a scavenger hunt, a riddle, spells it out with an algorithm
Hufflepuff: middle of an open field at sunset, out of the blue, in front of friends and family, proposes with a substitute ring (gum wrapper, ribbon, etc.), Netflix and Marry Me, a trail of flower petals leading to them kneeling, a cake shaped like an engagement ring, on Valentine's Day, with a pun
Slytherin: fancy restaurant, a work related event, large family gathering, tricks their partner into thinking something completely different is going to happen and then BOOM they're down on one knee, traditional style of their culture, after a long argument