There have been surveys around this with the top 20 coming in 2020 below (personally think bacon fries / scampi fries should be higher)
Walkers (Cheese and Onion) – 38 percent
Monster Munch (beef) – 34 percent
Walkers (Salt and vinegar ) – 31 percent
Pringles (original) – 28 percent
Walkers (Ready salted) – 27 percent
Hoola Hoops (beef) – 21 percent
Bacon Frazzles – 20 percent
Salt and Vinegar Squares – 20 percent
Skips – 19 percent
Monster Munch (pickled onion) – 18 percent
Quavers – 18 percent
Doritos (cool original) – 17 percent
Pringles (sour cream and chive) – 15 percent
Pringles (salt and vinegar) – 15 percent
Doritos (tangy cheese) – 12 percent
Walkers (prawn cocktail) – 11 percent
Nik Naks (nice‘n’spicy) – 10 percent
Scampi fries – 10 percent
Wotsits – 9 percent
Hoola Hoops (salt and vinegar) – 8 percent