At the moment, I want to say Xerath is possibly the worst champ in the game. Certainly one of the top 3 least picked. I have no idea what a complete rework would entail, but because the game is changing to have even more aggressive mids who get up close and personal, Xerath is becoming more and more outclassed in the meta. He needs to be way more mobile.
Everything feels better fresh out of the dryer. Even death would be nicer if it was dryer fresh. -UnitOmega010
There's nothing wrong with his kit. If an enemy champ gets close enough to you that mobility is a problem you've done something very, very wrong. His 3 second CD Q and spammable ult make him a prime candidate for BFT (or whatever the SR equivalent is) in S3.
Just because a champ doesn't fit the meta doesn't make him bad.
No, top 3 are Urgot, Karma, and Sejuani. He's in the top ten though.
Xerath is becoming more and more outclassed in the meta.
Here is the thing, it's not that Xerath is outclassed in the meta, but rather that he doesn't fit the meta. Tristana is outclassed in the meta because the meta for bot lane is people with good poke, escapes, and burst damage, and her poke/escapes/burst damage are inferior to Graves/Ez/Corki. AP Sion is outclassed mid because the current meta for mid are people with high mobility, good farming, and great ganks that can transition into either a carry or utility role and Sion isn't mobile enough and doesn't have carry or utility potential.
Xerath fits the poke/seige meta. He's probably one of the best mids at that, next to Kog'Maw or Anivia or Lux. But he while he's really good at doing that, he can't fulfill other mid roles because the thing that makes him good also stops him from moving for several seconds. So whenever the meta doesn't support mids that poke/seige, Xerath won't be picked.
Is it bad that some champions are specialized for certain roles? Eh. I don't have a problem with it. I like the idea of champs having specialized roles because it makes the ban/pick part of draft far more interesting when you can pick/ban play styles instead of just picking/banning whomever is OP atm. However, I'm not a part of Riot's design team and they seem far more interested in everyone being interchangeable.
Honestly, changing every champion to fit the current meta is bad for the health of the game. It's different when a champion does not fit any meta (Karma), but ideally Riot shouldn't be trying to coddle the meta.