Stormbreaker ups BRB's durability showings CONSIDERABLY. He's standing stock in the middle of planets goiing boom, without taking a scratch, fighting surrounded by cosmic energy all over the place against Stardust and Asteroth, and most importantly being flattened by Big G and surviving... The planet busting stuff was impressive, but the Big G attack is what puts Bill's durability over Thor's, imo.
And yes, Superman and Captain Marvel also have highly impressive durability feats.. The question, is whether they can take Bill out before he pulls a Deus Ex Mechanica out of his hammer.