RE: Favorite or least favorite Pokemon Trainers (In-game)
Well my 5 favourite relevant characters (sorry Joey, I just don't care about your Rattata) would have to be:
Cynthia- I found her to be the first interesting champion since Blue (Steven isn't necessarily the canon champion for Hoenn, in my opinion, so I ignore him for that, he is still one of my favourites though). She takes such a proactive role in the games and she is always portrayed as an incredibly strong trainer. I also liked her diverse team (I dislike single type champions) and the fact she has one great signature pokemon that fits her character well.
Alder- He has a similar role to Cynthia in the games, although a bit more on the side of elderly grandfather mentor instead of crime fighting lady. But again, I prefer him as champion over Iris. Iris bores me to tears, honestly. I think they could have picked a better replacement for Alder in BW2. I also like his usage of Bouffalant and Volcarona. Those pokemon are perfect for him.
Karen- I love her design. It doesn't necessarily scream "dark type master" but once you know she uses dark types, it's almost a perfect matching look. I wish she had a bigger role in the games, but still, the first dark type master is pretty good. Also, Houndoom. It's a great fit for her. And how can you not love Houndoom?
Steven- Even though I don't count him as a canon current champ (former perhaps, I think it might have said at one point in Emerald actually), I still like his character. Good, solid design, and great type choice. Even though he was a single type champion, it's a hard type to fight against. So much defensive power and then with his heavy hitting Metagross he was a challenge. And that Metagross was a wonderful signature pokemon for him.
Pike Queen Lucy- I like her design and usage of serpentine pokemon, which are some of my favourites. I wish in the games they had made the frontier heads a bit more important (maybe run into some during the story for minor quests?) I also love her signature pokemon of Seviper. Poison types are my favourite, and Seviper just feels right with that wicked blade tail.