Nah varus has one of the best ranged ad ultimates - You put varus leona bot. Dat cc
Anyways graves kog and urgot are the best ad carries atm vayne loses laningphase always, if the other carry knows how to play
But hey you must know varus is bad while my friend went from 1.9k to almost 2.1k by just playing varus and urgot carrying erry game
Varus' ultimate sucks. If it was a spreading stun, it'd be better. The fact that it's a snare allows mages to simple burst him down because his range isn't the "best"
Kog is one of the best because of the damage in % of your health is in his kit. That makes him a tank destroyer, while being able to kill anybody else with relative ease; while having a huge amount of range with his W and R.
Urgot is definitely not one of the best. He cannot do good unless sufficient farm and a decent amount of kills to scale into late game, since he's early game is his strong point. The only really, really good aspect about him is his ultimate, which can single out any threat easily if caught off guard.
The reason why Vayne is so good is simple; true damage. Have that in a kit itself in the RTS genre is just a bad itdea because the True damage means a few things: She takes out squishy targets easier, She takes out tanks easier then Kog (If a high enough attack speed, but that's a risky build). Now another reason why she's so good is her kit. She can Q to dodge some skillshots with ease on a good player, she can knock people away or stun them against walls with her E which can be a free kill, an escape, or an easy stun lock on a threat during a team fight. And her ultimate is all the better because it's makes her passive better while amplifying damage, which makes her chase no problem and gives her a ridiculous amount of damage stacking with the true damage. If she's doing bad, she can farm just slightly and everything is better simply because of the true damage.