Why they're at the top.
When you hear: Assassin. Fighter. Support. Mage. Marksman. Tank.
You think:


Roles I don't see a problem with: Support, Tank. All are either even choices or just not reliable against the other possible roles early on so they can't get strong before the game ends. Mage as well, but that's because most mages are pretty even, or completely terrible, and the ones that are too outmatched to play mid are played as support.
So assassins, fighters, and marksmen. (for my purposes, jungler isn't in here because that is almost completely limited to assassin and fighter right now) I'm not going to rant about how the ones I listed, and any ones that can stand up to them need nerfs. No. I'm going to address the real issue: Other champions can't stand up to them.
When you think fighter,
is a prime example because he has everything a fighter wants: sustained damage, good clear, excapes, and a little cc.
is just as good because she has scaling HP% damage that's usually reserved for tanks, two gap-closers, and some CC. Issue is there are tens of champions who DON'T have everything a fighter needs to be able to go toe-to-toe with them.
As for Assassins, right now the top assassins can go in, delete an enemy, and return to safety or delete some more. There are fewer assassins than who can't achieve this than there are fighters who can't stand up to the top fighters, but the fact is
and
are unplayable because one doesn't do the damage in a teamfight, where it needs to be done (due to a mechanic designed for killing people caught out, which shouldn't happen at the top levels of play). The other almost completely lost her cloaked playstyle and relies too heavily on a single spammed move with not much damage. (other examples exist, but I'm naming two that are either at the top or bottom of each.)
Right now the focus of marksman is somewhere between being insanely mobile and dealing damage they shouldn't deal. Point here being that
can't fight any of my examples because of their mobility and on-hit damage,
doesn't work as a marksman or a mage because his doesn't affect dashes and his high range doesn't mean anything against their superior on-hit damage (or extra auto, or infinitely stacking button-press execute).
We see the focus on being extremely mobile and high-damage recently. That leaves champions like
(Yeah, MS doesn't mean much against dashes recently) in the dust.
There are 80 champions they're hardly looking at because LCS play doesn't give them information to change them, because no one in LCS chooses them. They are neglecting the variety of the game by forcing a pool of less than half the champions onto players who don't want to see the "Defeat" screen every game.
I'm not saying the top ones need nerfs, but everyone else needs compensation. (And I'm not talking the kind of "compensation" that
got for the removal of DFG)
TL;DR: Too many champions are ignored for buffs because they aren't meta. They all need to be reworked to fit the direction the game is going in with its new flashy play.