The Hulk is one of my top characters but I hate the 'unlimited' aspect of his strength because after a point strength really does mean much as those in the top echelon of strength usually have cosmos/magic powers that make strength redundant. So here is my attempt to bring a little rationality to his strength. But first the assumptions :
A writer can make anything happen, I know.
The use of science here is not hardcore, it's just for fun.
The Hulk gets stronger through increased stress, which most only choose to use anger. Taking only maximum anger as a driver, it can only truly manifest itself in insanity, which has no limits (like bloodlust).
Now the Hulk's cells get denser from extra-dimensional gamma radiation. The more angry... the more gamma...the more density. Surely then the limit of the Hulk's strength would be when he turned himself into a black hole. If you compressed the earth to the size of a pea it would become a singularity. So, crappy writing aside, this is technically a limit.
But just imagine he stops short of this...the qualities he would possess would change a lot of battles...such a pool of gravity would slow a lot of people down...he could border a neutron star, quasar or some other entity.