If I were a Trump supporter I'd say Pennsylvania. If I were a Biden supporter I'd say Michigan. Both of those states depend on turnout. It's important for Biden that college students in Ann Arbor and AAs in Detroit show up, since it's unlikely that Muslims in Dearborn are going to. If not then it'll be more likely to be a Trump win. For Biden it'll depend on Eastern PA. If they show up, he should win it. The main reason Clinton lost it was due to lack of enthusiasm in this part of the state, while Trump had no problem with turnout in Western PA. If the polls are correct that Trump could win Iowa by nearly 20 points, then he should easily win WI.
If Black voters in Detroit don’t show up, it’s hard to see them show up in Philly either so Biden’s likely cooked in PA as well if he loses MI.
Also, if Trump’s won Wisconsin he’s likely won the election unless you think Biden’s in better shape in Arizona than polls suggest, and MI/PA are likely just icing on the cake for him.