If full builds mid, would amumu be the best?
As a tanky ap mid, assume both teams got to full build. Wouldn't he be a great ap mid? % damage and tank unlike ONE or the other. Good ult = gg
As a tanky ap mid, assume both teams got to full build. Wouldn't he be a great ap mid? % damage and tank unlike ONE or the other. Good ult = gg
Amumu doesn't get immediately tanky for free. He just has strong CC.
If you want reliable CC, %Health damage, and actually strong laning in your midlaner...

Yeah but I said assume everyone got full build somehow
And that doesn't change the fact that full AP Amumu really only exists for his ult, which...does just about as much damage as Malzahar's Q and only works if he throws himself into the middle of the enemy team.
AP bruiser Amumu works well due to items like Liandry's, Abyssal, and Rylai's turning him into a perma CCing, tank-shredding sobbing juggernaut. Full AP just gets nuked down before he can even engage.
Amumu as a damage source is pretty strong. That said, his kit isn't really the best for laning, so it's unfair to judge his potential as a mid laner only on the assumption that he reaches full build. Like how vayne or jinx are probably (some of) the strongest late-game adcs, but they can get bullied by stronger early-mid game champs like graves and corki, so they maintain a pretty normal level of power.
full builds poppy or susan > everything else in this game