Argonians hands down.
My reasoning for this statement is only one, Black Marsh. Argonians are the only race that can live and breed freely in the Black Marsh, which, except for a select few places, is almost uninhabitable for the other races. No other place in Tamriel has this kind of situation.
Firstly, the marsh itself hinders any form of movement, making it much harder to do basically anything. Living in a marsh is like living in a place with gravity higher than that of the Earth. So the people living there, i.e. the Argonians have to be extra strong just to carry on with their daily lives. Skyrim has harsh weather, true, but it can be conquerable easily by just changing your attire, something that is not an option in the Black Marsh. To give you a perspective, you can see Argonians in Skyrim, from its big cities to even the remotest of places. But you will never find a Nord taking a morning stroll in the marshlands of Argonia.
Secondly, their scales. As a wise Whiterun Guard always said, "The best offence is a good defence, eh?" The Argonians live in the marsh, and due to its unique properties of making life difficult in every step, wearing heavy armour in combat is just not feasible there. So they have scales, scales so hard that they can forego the need of heavy armour entirely and fight only wearing leather armour. To me, strength is not just how hard you can punch, it is also how hard you can punch without breaking your wrist in the process. And Argonians excel in that department more than any other race in Tamriel.
Lastly, their tails. But Krosis, what does a tail has to do with physical strength? Well, take it from a dragon priest, everything. A tail improves a creature's balance manifolds, which means you won't reduce the power of your punch while losing your footing in a fight. An Argonian (and Khajiits, for that matter) can punch with the full weight of their body behind it, which is physically impossible for their tail-less counterparts, as the action would carry them over forward and just serve to make the punch utterly useless.
That is my opinion anyway.