Who is the strongest hulk?

The previous answer, "earth," while (mostly) true, is rather inadequate. Though the Hulk appears as a transformation of Bruce Banner, which could happen anywhere (even in the vacuum of space), the first transformation occurred in a military infirmary somewhere in the New Mexico desert after Bruce saved Rick Jones from the blast of a gamma bomb that he had been working on (which, in turn, exposed him to gamma radiation from the same bomb).