Leon, without a doubt. He seems to have garnered a lot of hatred for his use of charizard, but I love the fact that we finally have a Champion that is not only widely-beloved by his region's people, but also... a Champion in personality. It always struck me as unusual that nobody- the protag included- recognised people like Cynthia and Diantha as the Champion, since I always figured that the position was a pretty big deal, especially given that it's the end goal of every main story of every main-series game. He's delightfully cheerful without being a total joke, and is actually a competent adult, encouraging you to continue your Gym Challenge while he, as Champion and as an adult, deals with issues that in other games, the protag would be forced to solve. IIRC, the only time during the main story that you have to deal with something yourself when he's there is Eternatus, but it's confirmed that he was badly injured and therefore out of the picture.
Other than that, I love Raihan. He's such a unique character and I think he's hilarious. Plus I love that more than just being a dragon leader, he's a doubles battler and a weather abuser. It's brilliant.