E5, C5, C6, E6, D5, D6, Nf6, Nc6, G6,
If you are going to play E4 as white then learn the variants of those black counters. Pretty much in that order.
Spend most of your time on E5 and C5 though. Those get played the most so that is the first tier. Next look at C6 and E6, Second tier. Spend a little time on the rest. They are third tier and not generally very good responses to E4 unless black is just a better player than white.
I started with E4 and that is the system I used for studying that opening.
variants