Iris was hard because someone let the programmers (or whoever decides what the movesets/items for trainers will be) on Smogon in addition to the high levels. Cynthia is still hard no matter how many times I replay a Sinnoh game - that Garchomp is a beast, and her other pokemon aren't any slouches either.
Easiest...I think I'll have to go with Blue. By the time you get to him, you already know his team pretty much backwards and forwards and which of your pokemon most easily counter each of his.
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Heart of Crystal - a Crystal Nuzlocke challenge story
"He had it comin'! He had it comin'! He only had himself to blame! If you'd've been there, if you'd've seen it, I bet you you would have done the same!"
I don't see how everyone found Lance easy unless they did some intensely mind-numbing grinding. He was literally the ONLY champion I did not beat on the first try.
Well, for me, I didn't level grind particularly. I just solo'd the region with only my starter and by the time I got to the Pokemon League my Typhlosion OHKO'd pretty much anything in his way. I took care of Lance easily with the monstrous level 80 Typhlosion ... so yeah. No level grinding. Just a result of over-using and only-raising your starter
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I beat Iris fairly easy for me I had to use healing Items but I effectively only had 4 Pokemon a little more training and I would have crush here. I was really close to not having to use items against her.
There is patience and then there is sloth.
Easiest: Either Lance or Iris. Lance because five of his Pokemon have quadruple weaknesses (as long as you have Electric, Ice and Rock type moves, you're fine). I don't know why, but Iris was incredibly easy as well. She just didn't seem that hard to beat.
Hardest: Cynthia. I hate that Milotic so much - last time I played Platinum I just got Golem to blow up on it.
I didn't find Iris as tough as you all are saying. Rly, she beated two of my pokés, but nothing to complain about. The fact that most of us thought "lol my team is around lv 55 before the league, better if I train everyone to 60", or at least that's what happened to me, doesn't help either.
Now, Cynthia attends the concept of Champion. I would love if every Champion had the same team balance and AI that she has. Also her BGM, whoar. @_@
For the hardest, I would have to say Lance in HG/SS, mainly because I walked into that battle thinking "Oh, he's gonna be a pushover like he was in Gen II." Boy, was I mistaken. His Gyarados almost wiped out my team. In close second would be Cynthia. Her Garchomp didn't give me any troubles, but her Milotic wiped the floor with me.
The easiest one for me would have to be Alder. I didn't lose a single Pokemon when I went up against him, and I sweeped his team with my Excadrill.
For me, the hardest was Cynthia, at least in Platinum. Honorable mentions include Wallace (Milotic AND Tentacruel!?), Steven, and Iris (though, if my Lucario would've been just a little faster...).
Easiest is either Blue or Lance. An Ampharos and a Feraligatr with Ice Punch will destroy Lance, and Blue... well, you're already prepared for him.
Honestly, so far the easiest champ I've faced is the most recent one, the BW2 one. I was actually healing up for the Elite 4... swept the champion. I think I used like one potion the whole time. Mostly one hits. It was redonk.
Alder was hands down the easiest Champ, his team shared too many common weaknesses and his Volcarona's moveset was awful. You can't get more counter-productive than a Quiver Dance followed by Overheat :\
As for most difficult, I always had trouble with Wallace. His team was well equipped for dealing with Electric and Grass types, and once your Water counters were crippled he became quite difficult to beat, especially that damn Toxic-stalling Milotic.
Hardest: Cynthia. Took a darn 105 tries to beat.
Easiest: Alder: Just wasn't hard, I guess.