Kendrick is definitely top tier. I don't think there's a 'best' rapper under 25, just people who are clearly above others.
Referring to your comment about Wiz, how is he that far from Kendrick, they're completely different. It's not just about rhymes, it's also about the rappers flow, beat selection, and people they work with. If you ask me, Young Khalifa raps over some of the phattest beats created. I'm not saying Kendrick doesn't, I'm just saying that you shouldn't be so quick to wipe other rappers from the drawing board.
In my 'top tier' of rappers under 25 (No order):
Kendrick Lamar
Wiz Khalifa
Curren$y
Earl Sweatshirt
Mac Miller
Machine Gun Kelly
Styles P
A$AP Rocky
Joey Bada$$
But if anyone is going to be compared to Kendrick's writing ability mixed with flow, it's gotta be Donald Glover/Childish Gambino.