Needs a bit more description of the creak such as when does it happen. Your "extreme pressure up a hill" indicates that it goes in time with the pedalling. You can try a number of things to help narrow it down if that is the case. Stand off the saddle? Creak gone, try greasing the seatpost and making sure the saddle is correctly bolted down. If its a Brooks saddle do the tension up a bit. Take your weight off your hands if you can? Creak gone? Look at the bars mounting and the headset. If neither and having checked what you've checked, make sure the bottom bracket is properly done up, that the pedal spindles are greased where they screw into the cranks and if they are spds that its not the rubber on the soles of your shoes creaking against the pedals. If you've eliminated all those look further afield like the rear wheel axle/mounting and also inspect the frame and forks very carefully for cracks. If all that fails, buy some earplugs.