Assuming this is when both are at their strongest, Bill is likely to be more powerful. Now, it’s likely that Ultron would get a few good hits in before Bill starts to take him seriously, but given how advanced Bill’s powers are, I don’t imagine him losing. Since Ultron has the mind stone, Bill might actually be able to go into his mind and pull a Zola on him, or maybe he could deactivate the infinity stones entirely, or some other thing that makes use of the fact that his reality warping is superior to Ultron’s.

So we got ultron who lost to a virus and bill who lost to two kids and a old man

@Hazbinhotelanddcfan Let's not undersell Bill here. He lost to two kids and TWO old men

Honestly here bill’s defeat is more embarrassing cuz like he got defeated by people who don’t even know how to fight and I’m pretty sure the average human can beat most gravity falls characters (excluding the monsters)