Thanos, if you check the Official Handbook of the Marvel Universe, can press about 50 tons. Making him half as strong as Hulk, Thor and numerous other heavy-hitters. He can use his energy powers to increase his strength, but it takes a lot out of him.
Galactus has potentially limitless physical strength, because he can amp it up by using his power cosmic. But he rarely depends on physical strength, when all he needs to do is think something and it happens. However, while he contains a massive amount of power cosmic, it's still a finite amount. Silver Surfer can also enhance his phyiscal strength with his power cosmic but, like Galactus, rarely does so. Even still, without enhancing his strength he's still in the Class 100 range.
Thor has the ability to enhance his strength. Already in the Class 100 level, when he goes into berserker warrior rage his strength increases ten-fold.
Hulk's strength is potentially limitless, because the madder he gets, the stronger he gets. And since there's no way to quantify the limit of an emotion, you can't really quanitfy the limits of his strength, either. In the first Secret Wars, he was able to hold up an entire, 4 billion-ton mountain range. And that was as "Smart Hulk," meaning he had the Hulk's body but Bruce Banner's mind, and was only mildly irritated at the time.
However, there is one thing than can elevate a character's strength by a factor of 50 -- the Captain Universe force. When Spider-Man had it, it made him strong enough to punch Hulk into orbit. So imagine if someone like Thor, Hulk, Juggernaut, Colossus or Thing were to become host to it. Though it typically only binds itself to those worthy of it.
At any rate, I'd say Hulk is the strongest simply because his potential strength is limitless.