10 Most Powerful Beings Venom Has Defeated
7. The Hulk - What If...? #4
Like the new Disney+ it was based on, the original What If...? comics existed basically to ask questions about various points in the Marvel universe to see how things would have played out had events been different. Spoilers, they often had less than happy endings.
In the fourth issue, for instance, the question was posed "What if Peter didn't defeat the black suit, and the symbiote wreaked havoc across the Marvel universe?"
TL;DR, nothing good.
Venom becomes absolutely unstoppable without Peter to hold it back, and only gets stronger as the story goes on, starting when it killed the strongest Avenger in the most efficient manner possible. See, Venom did away with Spidey by draining away his life force, killing him immediately. Venom then did the same thing to Hulk by bonding with him and draining away all the gamma energy from the monster, taking away all of Bruce's powers and transferring them to Venom.
Fortunately, Venom's famous weaknesses still remained so one merry greeting from Black Bolt managed to finally put the thing down. Which is good, because Venom was about to do the same to Thor.