Xerath is an interesting champion in a team's composition because he has excellent poke capabilities, but also has an excellent team fight presence. This is in contrast to a champion like Nidalee, who when built AP has arguably the best poke in the game, but if built is very weak to hard engages.
In my experience, champions that work good with Xerath if you're going for a POKE
Whether or not it's ADC Ezreal or Blue Ezreal, his ability to constantly poke with his Q compliments Xerath's ability to poke with his Q. It works very well together since the damage is split between physical and magical.
Janna is the best counter-initiation champion in the game. She can push all of the enemies that just used cooldowns to engage on you away, allowing you to reposition and start bursting.
Similar to why Ezreal works so well with Xerath, his poke is as long range as yours and is physical to complement your magical poke.
If built AP, she has a slower poke that does more damage. If built bruiser, she can be a good meat shield that can jump on the enemy carry and is resistant to their poke while still having decent damage on her own poke.
Similar to Ezreal and Jayce in terms of the range and nature of the damage, but Varus also has an excellent ultimate for either disengaging or catching an enemy.
Champions that work good with Xerath if you're going for a TEAM FIGHT
Roots everyone in place and his primary target will have reduced MRes due to his passive. This allows you to burst someone without worrying about them dodging your damage. It also allows you to burst a specific target extra hard due to Amumu's passive.
Diana is great at initiating and steering attention away from the back line. She even has an ability that clusters enemies around her so your ultimate will hit as many enemies as possible.
Similar to Amumu but much higher damage and much more lethal to the primary target. Fiddlesticks's passive is amazing when complimented with your already high MPen, and his fear almost guarantees a kill, as long as the enemy being feared doesn't have a QSS.
He hard initiates like a champion and can soak up a lot of damage. While his AoE is on you can keep him alive while he keeps enemies away from you.
His initiate is just as good if not better than Hecarim's. He forces people to either burn a Flash, use their escape immediately, or die.
He can run in and stun the enemy team, allowing you to hone in on one specific target. If the enemies are clustered you can pull off a veritable 2v5 with the combination of your ultimates.
She can initiate from afar and pin down and catch someone very easily. She can't lock down the entire team, but she can lock down a single person extremely well.
One of the fastest, most reliable initiates in the game. The more people it hits the easier it is to get multiple casts of your R on a cluster of enemies.
Her ultimate has a similar function to Kennen's, but is easier to land and allows your team to run in faster while you stay back and burst. She also has a bubble to keep people from killing your back line.
Very reliable initiation with his ult. He's also capable of locking down a single person better than anyone else in the game. He's a big meat shield that can initiate amazingly well and counterinitiate amazingly well.
Excellent initiator. He's great at drawing attention away from you important champions and towards himself. He can peel decently well but is lacking in counterinitiation.
His ultimate is good for slowing people down to land your spells and also does lots of damage. If used properly Rumble can cut off enemies from each other so you can kill one person and then turn to the rest of their team.
Similar to Nautilus, but has a better ultimate for initiating since it stuns multiple people instead of just 1. She soaks up a lot of damage and slows down the enemies so they can't reach you before they die.
The more people she hits with her ultimate, the better the fight goes. Since her ultimate draws people into her she can absorb a lot of the aggro as well as force the enemies to cluster for your ultimate.
Very high autoattack range mid to late game lets her stay slightly in front of you, but she's super safe since she has both a jump and her AoE knockback ultimate which will keep both of you alive.
Has an AoE snare as well as an AoE knockup with decent damage on both. Can either peel multiple enemies off of you or set you up for easy bursting.