Gorosei power classification should work like this
The closer they are to the sun, the more powerful they are.
Since the earth is useless in this ranking because no gorosei has its name we have topman, nasjuro and marcus as number 1, 2 and 3 which corresponds to the classic oda scheme of luffy vs number 1 zoro vs 2 and sanji vs 3.
Some might say that luffy is going to face imu directly, which seems absurd to me. Luffy will obviously beat a gorosei as he's always done with every branch of opponent we've ever had ( yc, tobiroppo, yonko, admiral etc... )
Some may say that sanji will never face a gorosei, just as he wasn't supposed to face any yc in onigashima and only page one
Venus and mars are often pictured next to Topman too, which may reinforce the fact that they are numbers 2 and 3.