This is definitely a good question :3! I found the concept of stat trainers interesting and I enjoyed their appearances in the game, it always made me satisfied when I helped one out and sometimes you get a nice reward in return as a thankyou gift! I would definitely love to see this return in future games and it's interesting how you pointed out that they specialise in a certain stat, I actually never realised that haha :3. They definitely however made great companions, I loved their personalities and designs, their teams were also interesting to fight with!
My favourite stat trainer? I don't honestly I think I have one but personality-wise, I think I particularly liked Cheryl, she was a kind, warm-hearted woman who I found hard not to love, her appearance definitely fits in with Eterna Forest, with plenty of green which could resemble nature,trees, plants, etc.
I also loved the others, they were special in their own way although I think my least favourite was probably Marley, she seemed a bit out of place and awkward in the game for some reason, why was she at Victory Road? Why didn't she ask other trainers for help? etc., but then again Mira was in a similar situation...but still her appearance wasn't one I was too fond of. Back onto favourites, I think my second choice would definitely be Riley, I've always liked characters with calm and cool personalities, over those with a heroic and reckless ones, in my opinion. And Riley seemed kind enough, he also has the bravery of a hero to stand up to Team Galactic so I admired that as well and also his suit is pretty cool! :D.