Who cares about king of equals? Every pirate legends of Rogers era are his equals including his own no.2 and3

The strongest 6 in this island at this era logically should be
Xebec
Roger
WB
Garp
Shiki
Ray etc
But Oda wants to use the 6 relevant top tiers and whom are represented as a measuring stick of strength in the show to combo attack Imu (so as to hype Imu as well)....... irrespective of someone among the 6 should be alot weaker and won't make sense to be with them.
