How I rate the different allergens:
Pollen: 10/10 - they're fine getting ready in their house that morning and then BAM. Step out the door and start sniffling, then their eyes water, their nose starts running, and once the sneezes are coming out it's too late to turn back and go home to grab a pack of tissues, they're gonna be sneezing all day at work/school or wherever they're going and they're completely surrounded by it.
Dust: 9/10 - only gets a point off because it's not as frequently available as allergens in the outdoors, but it's the perfect recipe for a suddenly sneezing fit, especially in intimate quarters. Dusting off an old book, working as a maid, going into the attic, staying in an old cabin, pulling a blanket down from the top of their closet, and suddenly they're consumed in a mess of sniffles and sneezing, hurriedly excusing themself for their allergies between hitching breaths, and begging for a tissue.
Grass/hay: 8/10 - similar to pollen, they step outside and are overcome by their allergies, but with the downside that people don't usually give grass as a well-meaning gift, but also the upside that going out to the countryside or staying on a farm will completely wreck them, and if they're not used to being outside the city they might greatly underestimate the power of their allergies in another environment.
Perfume: 7/10 - now is basically being allergic to someone, and your allergy is in their hands. Plus it lends itself to great settings, like being over at their house, or near them in the changing room, and it gets worse the closer you are to them. Smelling their hair, borrowing their clothes, or best of all: them giving you a makeover complete with a spray or five of their favourite perfume until you're a sopping sneezy mess.
Fur/dander: 6/10 - I'm not too interested in having pets in proximity to my sneeze fetish content, but it's pretty nice when someone stubbornly denies an allergy in order to get or keep a new pet, or they visit a friend's house who unexpectedly has a few cats that shed everywhere and they suddenly can't hide their allergies anymore. And it gets on their clothes, and it's in all the bedding, and even the towels, and now they're sneezing all over the house and talking with a stuffy nose.
Detergent: 5/10 - this is similar to perfume or dander, just more specific, but it is really sexy when they hop in bed and realize they're allergic to the new laundry detergent that they just washed EVERYTHING in, or they're over a friend's place or hotel and too embarrassed to mention they're definitely allergic to that brand, and have to endure sneezing up a storm all night.
Pepper/spices: 4/10 - also very specific, but has the upsides of being involved in food which is great when you like contagion and for desperately trying to avoid contagion. Additionally, it makes for a great prank, and has a notable place in restaurant and cafe scenes, including on dates or upscale joints.
Mold: 3/10 - this isn't very sexy, but it has one redeeming quality in that your character can drive themselves crazy trying to figure out what's making them sneeze constantly at home, cleaning everything top to bottom several times a week, and finally resigning themself to being a helpless sneezy mess.
