I have not battled Iris as champ yet but for the ones I have, I have to say Cynthia. Her Garchomp is a beast and her Gastrodon can be really tough also. And I believe her Pokemon all have perfect IVs.
As for the rest. Blue was pretty tough in blue, but as I became more experienced he got much easier. Lance was really underleveled IMO. Steven was super easy if you chose Blaziken or Swampert. Wallace was just annoying and should have never been champion, I mean you just battled a water specialist why make you have to face another. And lastly, Alder. I almost voted for him, because his Quiver Dance Volcarona is REALLY good, but a good rock move cripples him easily.
This is my thought. I may change my opinion after I get my Black 2 copy I am waiting on. 1 month 5 days!