Hmm, I do like many characters and Pokémon, but some of my favourites are probably the following:
Humans:
Ash (OS, AG, DP, XY): Yes, all these iterations of Ash exhibited different personalities and traits, but that's why I liked these iterations since it showed visible character progression and growth, but at the same time, you knew it was still Ash. He started off as a naive, arrogant and hotblooded savage in OS, before slowly calming down a little in AG and making his first real strides as a competent battler. In DP, he finally showed how far he came as a trainer and as a person, with his maturity and skills as a skilled trainer coming through. And that carried over a step further in XY, with not only Ash being super calm and composed, but it was finally his time to inspire others around him and teach them a few things.
Brock: He will always remain my favourite companion. His older brother dynamic was always great to see with Ash, and he visibly cared for Ash. Beyond that, the amount of humour and overall hillarious antics he brought to the show will never be forgotten.
Dawn: First Pokegirl to have shared almost equal amount of spotlight with Ash, and I loved her cheerful and upbeat personality. She and Ash really felt like best friends, constantly training with each other and supporting each other.
Serena: My favourite Pokegirl. There's something I really loved about her quiet, shy demeanor initially, before she gained confidence and huge amounts of advice and help from Ash to grow into a strong, confident person. She just seems like a really supportive person, one who'll always stick by your side even during troubles.
Alain and Mairin: Huge surprise, but I loved these two characters. I'm including them together since I feel they work best as a duo as they bring out each other's personalities even more. Mairin is the beginner trainer without a care in the world, while Alain is stoic and clearly troubled. Yet, I loved how Alain, despite that cold external demeanor, showed how much he cared about people who he really loved and valued, like Mairin and Sycamore.
Pokémon:
Charizard: Truthfully, as a kid, I hated what a massive dick he was, costing Ash the league. However, he's remained one of Ash's most loyal powerhouses. I like how he's a showoff, but clearly cares deeply for Ash and everyone.
Greninja: My favorite out of all of Ash's Pokémon. Yes, he's stoic, but I love that since it showed how much he respected Ash and shared the same desire to grow stronger and master the Bond Phenomenon.
Braixen: Another surprise addition, but I loved the closeness between Serena and her Braixen. She started off as an unlikable, snobby Pokémon, but she grew along with Serena once they found a dream they both wanted to pursue.