That would be Arizona (0.31%), Georgia (0.24%), and Wisconsin (0.63%), with 37 electoral votes collectively. The total margin in all those states combined was only about 42k votes.

If Trump had won all of those instead, there would have been an electoral college tie, which would have resulted in the House deciding on a one-vote-per-state basis. It wouldn't have been pretty.

Biden also won Pennsylvania, with its 20 electoral votes, by 1.16%.