the absolute best
Not the absolute best you can do in general - just the absolute best you can do with your right to vote.
The Two Party System means that either the Democrats' candidate becomes the president or the Republicans' candidate becomes the president - it's effectively impossible for a third party candidate to win, and the presidential power of the elected candidate does not get diminished just because less people voted in total. This means that if one candidate is more evil than the other - then the less evil candidate winning is the best possible outcome (out of only two possible outcomes) of the elections.
This does not mean that you can't do better in general. Just that whatever you do better is orthogonal to voting. If you are working on a mini black hole that you plan to unleash in Washington to erase the entire government thus eliminating all its evil, then on voting day you go and vote for the lesser evil and then go back to your garage and continue working on your singularity bomb. After voting. Because voting does impact - even if it can't magically fix anything - and voting does not hinder your ability to influence in more impactful ways.