It varies depending on which game I'm playing tbh. For example I'm in my first Black save since 2014 and really enjoying it, with HeartGold being the game I just finished.
I started out with Emerald, but Wallace just doesn't play much of a role and is somewhat diminished in my eyes by having already faced his apprentice as a gym leader. Steven is cool, but he's not a Champion to me. Wallace's Milotic was a beast though. Cynthia using both Wallace (MIlotic) and Riley's (Lucario) mascot Pokémon also made her seem a little less cool.
I found it a great twist that Blue was the Champion. With no reference to anyone beyond the Elite Four throughout LeafGreen I thought perhaps the originals didn't have one, so to beat Lance, whose Dragons certainly seemed like the apex of the game given their rarity, resistances, and power, only to find that my rival was waiting was pretty cool. The character was pretty meh though. Lance is really cool, Dragonite is one of my all time favourite Pokémon, but I think I prefer the villain / antihero in the manga to the games version.
So I actually think Alder is my favourite! The post may seem chaotic as I didn't realise it as I was typing it out, but I really like that he's shown to be vulnerable within the storyline, laying it all on the line against N but losing, rather than the most powerful trainer in the region refusing to help you battle the evil team "because reasons".
Honorary shout out to Red - boy has no charisma, but you've got to admire the commitment of funding the construction of his own Pokémon Center so he doesn't have to stray far from his mountain. The battle theme he shares with Lance is possibly my favourite in the games I've played (Gen III-V) too.