I think we can all agree the most important swing states this year, due to their combination of size and closeness to the national vote, are Pennsylvania, Ohio, and Florida. Any candidate who can win at least two out of these three will have a good chance of getting elected, and a sweep of all three makes victory almost assured, as I will lay out below. I'll outline the most plausible victory scenarios for the candidate who loses a majority of them which will help make clear their importance. I'm using the current fivethirtyeight.com polls-plus model ranking of states to help determine the rank order of the rest of the states on the D-R scale.
One scenario I won't consider, due to its implausibility, is that Trump wins Pennsylvania while Clinton wins Ohio. Pennsylvania has been more Democratic than Ohio for decades, and there is no reason to think that won't be true again this year. The states are demographically similar enough that the odds of them moving in opposite directions over the next two months are remote.
If Clinton sweeps the Big Three, she'll be extremely likely to win. If we assume that Michigan will also continue to be more Democratic than Ohio and thus that Clinton will also take Michigan (an even safer assumption than Pennsylvania IMO), Trump would have to pull off this nearly impossible map (he'd probably win at least Maine's second district and possibly the state itself as well but both are unnecessary):
If Trump swept the Big Three, on the other hand, he'd be very likely to win, though Clinton's chances are a little better of surviving such a sweep than Trump's would be. She'd have to create this unlikely, albeit not entirely implausible map:
Where it starts to get interesting is if the Big Three split with two for one candidate and one for the other.
If Clinton won Pennsylvania and Florida, but Trump won Ohio, Clinton would still have a pretty solid advantage. Trump would need the following map, which while not inconceivable is pretty unlikely (Nevada would probably go for Trump also but would be unnecessary):
If Trump took Florida and Ohio, while Clinton won Pennsylvania, the election would probably be very close, and could go either way. Clinton would need this map:
Clinton winning Pennsylvania and Ohio would still leave her firmly in the driver's seat, even if Trump won Florida, although Trump has a better chance than if he won Ohio instead of Florida. He'd need this (he'd probably win Maine's second district, but wouldn't need it):
Finally, Trump's uniqueness as a candidate makes possible that he wins Pennsylvania and Ohio but that Clinton wins Florida. Clinton would have to be considered a narrow favorite if this happens (which helps show how important Florida is--the biggest of the Big Three). She could win with this map:
