Technically, if legendarys could have egg, they would be the highest. Pokemon like:
All of the pokemon with 680 BST, Celebi, Deoxys, Jirachi, Lati@s, Regigigas, Cresselia, Darkrai, Shaymin Arceus, Genesect, Thundrus, Landorus, Tornadus all require at least 30855 steps.
Second would be all of the trios except for the flying trio(Thundrus, Landorus, Tornadus) with 20655 steps.
Third would be all of the pseudo legendaries, Happiny and evos, Lapras, Munchlax, Snorlax, Unown, Wailmer and evos, Relicanth, Phione, Basculin, Alomomola, Axew and evos with 10455 steps.
Fourth would be eevee evos and aron evos with 9180.
PS after this the lists are going to be big so I'm just gonna list the number of steps.
Fifth is 7905 steps
Sixth is 6630 steps
Seventh is 5355 steps.
8th is 4080 steps
9th is 2805 steps
And there isn't a tenth.