I separate stouts and porters into the traditional smokey/coffee tasting and varieties with added flavour (usually fruit) which there is a bit of weird snobbery about.
Anyway as far as easy to get from a supermarket, I can vouch for:
Guinness West Indies Porter (already mentioned and at 6% ABV it’s got one foot in stout territory)
Saltaire Triple Chocolate Stout
Youngs Double Chocolate Stout
Shepherd Neame Milk Stout
Fullers Black Cab
Not as easy to get:
Brew York Imperial Tonkoko
Bollington Brewery Oat Mill Stout
BrewDog Tactical Nuclear Penguin
Some of the nicest I’ve ever had, I can’t remember the names of 😀 Imperial, pistachio, hazelnut and cherry stouts, ooommmh lush!!!