The iris that is the best at resisting weeds is the one that grows behind a weed barrier, at least in my yard. And I don't mean fabric spread horizontally across the surface, but a vertical barrier. We have a lot of quackgrass, and I don't plant much without getting it out first and making a barrier around the bed. That said, I would think that some of the older, really tough historics would be your best bet for growing on in spite of weeds.

Your problem may be because iris leaves are upright and do not shade the soil the same way as plants with horizontal leaves.

I don't know of any irises that shade the ground the same way as hydrangeas. Mulch is probably your best bet. It will prevent weeds and grasses from growing in your beds.