Iris has probably the best constructed team of any champion. There's actually internal logic to it. Aggron and Lapras function as hard checks to the traditional counters to Dragon, and are bulky speedbumps besides. Archeops and Haxorus are both built as cleaners, Archeops in the traditional sense, and Haxorus via being a setup sweeper. Hydreigon and Salamence are all around attackers specializing in Special and Physical respectively.

The team as a whole is built around forcing player reaction while leveraging damage output, and stonewalling attempts by the player to gain tempo. Any traditional counter to Dragon gets met by Lapras or Aggron, meaning the player isn't given free reign to spam Ice Beam. Then, Archeops either cleans house when you are out of full health pokemon, and your faster ones are already dead, or Haxorus comes in on a pokemon unprepared for it and Dances.

Iris is the strongest champ in the series by a huge fucking margin. Between the raw BST of her team, the thoughtful construction and role of each member, and the exploitation of general patterns of player team building she uses.
The best part though, is that Iris is nerfed HARD by playing on Switch. Her team is all about controlling tempo and punishing empty turns, which Switch mode takes completely out of her hands, meaning she's approachable for newer players. Iris on Set is a whole fucking new hell.

Compare Cynthia, whose team is full of dead weight. Garchomp, Milotic, and Spiritomb are the only pokemon of any real merit, and Spiritomb only because a bulky pokemon with Pressure is inherently good against a player that has a single overlevelled pokemon. Cynthia is infamous, because her team is tuned to counter gen 4's early route shitmons, the starters, and the box legends. If you bring something like a Gyarados, or a Steelix, take literally any steps off of Starter+Staraptor+Luxray+Box Legend+Lucario core, she crumbles. If you have an answer to Garchomp and Milotic, you beat her.