I'm gonna go with Cynthia. Playing through Yellow a few months back, I realized that Blue was really nothing. His movesets were crap so even if you were underlevel, you will still be alright. It's been a long time since I got to him in FRLG though, so I can't really compare the two, but it can't be much worse. Cynthia on the other hand was the first time since Blue that we had a Champion with a mixed team and no set type. And let's not forget the team includes Garchomp, Milotic, and Togekiss. Yikes.
Iris and Alder...well...they suck. Alder has a pretty decent weakness to Fire and Iris...I had no trouble with her in my Japanese Black 2, despite not even being able to read a thing she was doing. I still beat her without losing any Pokemon, and I was slightly underlevel compared to her, if memory serves. So, overall, in order of most difficult to least:
Cynthia > Lance > Wallace > Steven > Alder > Blue > Iris