Here's the things. I think Deadpool could win any fight if he REALLY wanted to. But usually his attention span and low tolerance for wasting his own time usually means he doesn't care if he wins or loses. Deadpool once said it best, "You may kill me first, but I guarantee I will kill you last."
What I'm basically getting at is even if he's fighting against someone like.....Thor.....He'd probably lose that fight 99.9% of the time. But his immortality and healing factor gives him an unlimited amount of do-overs and chances to try again. Eventually he's bound to win, whether by exhausting his enemy or driving them so crazy that they eventually surrender.
So do I think Deadpool could win a battle against some of the most powerful heroes in the universe?......No......But if he was willing to really commit to the idea of killing one of them (Which as I've already said is probably extremely unlikely), over time....I'm confident he'd eventually win.