was supposed to be, but I'm pretty sure she's just in a weak spot atm.
can be in the correct hands, with his map pressure, his being deceptively tanky, and good mix of both burst and sustained damage, I would call him one of the best, although his spell combos to do this properly are not easy to figure out by any means.
gets a fuckload of damage out of nowhere once late game hits, and just stalls to get there beforehand. Plus, with her egg, it's just that much harder to kill her, since you've got to do it twice.
was designed to be a scary lategame mage in the correct hands, but his primary damage comes from his soldiers, who attack with Azir's own basic attacks, so I'm not 100% sure if he counts.
's late game is fucking horrifying when played correctly; his MR shred, short CD Qs, and E when he has a lot of mana deal a fuckload, and he can still hit like a truck while dead, after which he will likely press R and kill a few people on the enemy team. But his mana costs are so high early to mid game that you don't see him much.
I'm positive that there's more than this too, it's just these are 5 deadliest late game when played well.