I realize this thread is super old but I thought I might write down my thoughts on the subject since I haven't done it anywhere else.
Overall, I'd rank the Champions like this:
Iris > Alder = Cynthia > Steven = Wallace > Diantha > Lance > Blue
I'm not including Red or N here as I don't consider them to be "Champions".
So, to put it simply. Diantha is not my one of my favorites, but she is not my very least favorite either. I actually didn't mind the fact that she didn't have a large role in the story, it just made it a bit more surprising when I found out that she was the Champion. I managed to stay unspoiled about it when I played through X back when the games were first released. Although, she was the one I expected to be the Champion, so it wasn't that much of a surprise. And at the same time, she does literally nothing compared to all other Champions which had some role in the story at least. Character-wise, she is lacking just like pretty much all other important characters in X/Y except Malva, Lysandre and the ones involved in the Looker missions. Her design is hit and miss for me, her dress is great and I don't mind her being dressed like a Fairy (or what it now is supposed to be) but I am not a big fan of her hairstyle. Her team was okay, though her main Pokemon happens to be my least favorite so that's a negative. Being able to smash it to pieces every time I face her feels good though. As for the difficulty... she was easy, just like all other Champions if you know how to play the game efficiently.
As for what I thought about the others... from favorite to least favorite:
Iris is my favorite. A young and energetic girl with a fantastic design and a good team, in addition to having the best battle theme and being the strongest in terms of levels when battled on Challenge Mode. I surely didn't mind battling her 36 times while playing B2/W2.
Alder is great too. His design, personality and backstory is great, plus his team is interesting and I like his part in the story in both 5th Gen pairs.
Cynthia is another good Champion. She has a great personality (I don't mind her backstory being shrouded in mystery), she is involved in the story in a great way and has a great team. Neat design too.
Steven is cool. I liked his involvement in the story, his personality and interets in stones and his team. OR/AS made most of these things even better.
Wallace seems (or at least seemed) to be very hated judging by many previous posts when I read through this thread. I found that a bit surprising (though most posts are over 2 years old so the general opinion might have changed). I liked him though. I didn't mind him being another Water-type trainer, he still managed to stand out compared to Juan.
Lance is one I have a hard time caring for. While he has a part in the story and not a bad personality or design by any means, I just find myself liking him less than the others. I remember when I played G/S/C that I just found it so surprising that he suddenly was the Champion when he had been a regular E4 member in R/B/Y. I liked him better as an E4 member to be honest.
Blue. The first Champion and my least favorite. As a Rival, he is a jerk with zero character development. As a Champion... he stays the same. That should have changed. I would have liked to see him change his attitude a bit upon becoming the Champion. He wasn't very involved in the story, only showing up at various places through the games to challenge you as a Rival, I don't like his personality either, and his design is the most generic out of all Champions. So he is my least favorite.