Well there is one big issue or several:
1: Who is strongest will all depend on the bias of the person/writer etc, and to fit a story, as we can see how strong/ability to take punishment shift from time to time in the same movies.
2: fictional creatures so its almost impossible to determine how strong they are, do we count only the movies, games or everything?
Im of the opinion a normal xeno drone/warrior would be able to lift and just beat a predator in strenght, like insects they are able to lift well beyond their weight. We dont see predator tearing down metal doors like the xenos, with their tech on we can see them being stronger but thats due to their tech so I dont count it.
Xenos can also take a lot of abuse and still go on.
Predator seem to depend on the story how much they can take, someone wrote it took 19mm rounds to hurt the jungle hunter and the los angeles predator could take shotgun hits and still go on.
But we saw in predator 1 that jungle hunter got injured from small 5,56mm assault rifle shells and he then struggled to run and we later see him having to operate and remove the bullet and was far from unhurt.
So 5,56mm rounds can kill them, so pulse rifle would if we take predator 1 movie as correct, be able to kill them with ease.
But then im talking from my army experience and shoting 5,56 and 7,62 and 9mm and bigger rounds on stuff and the effect xD
But in the end its up to each fan or writer etc to determine who is stronger, I vote xenomorph but im bias to them.
