What should I be considering when purchasing avocados?

I’m looking to become a pro at picking the best avocados. What aspects of the size, shape, color, feel, etc. should I be evaluating and how important are each? How does the timing of when they are most optimal to eat factor in—i.e. should I be purchasing them x days before I intend to eat one?

Help me decide not to run a giant science experiment (like this one) to figure this out…