Let's just say for the sake of it that they have a random encounter on earth while Hulk is in his WorldBreaker incarnation. Thanos doesn't see Hulk as a threat and insults Hulk as being weak and punches Hulk once and begins to walk away. Now WorldBreaker hulk panics at the power of Thanos and begins to go insane and gets very pissed off and runs at Thanos. Thanos turns around and is ready for what in his eyes will be an easy brawl. Who would win with sheer strength? And they also have no idea who each other are before this encounter.
Because Thanos doesn't see him as a threat at all. Also I don't mean panic in the sense of being scared of his opponent, just being confused of the power Thanos has. But interesting comment, I'd probably bet on Hulk too.
Thanos isn't going to take Hulk lightly.
He compared Hulk to the Power Gem, even stating as to avoiding direct confrontation with the Hulk.
And that was before Greg Pak's tenure on the Green Goliath.
If this is random and Thanos has no time fro prep, then WBH definitely will take it.
The best chance for Thanos, if he has prep, is to do what he did to the Power Gem wielding Champion.
Nonetheless, it's a task easier said than done.