Before Journeys unironically it's probably Brock, with his only competition being Misty and Clemont.
Cilan is also a gym leader but has worse feats and weaker pokemon.
Clemont showed a lot of strength against Ash during their gym battle even though he only barely trained beforehand, meaning he was around as strong as Ash for over half his gym challenge (and XY Ash was REALLY strong).
Misty's not as knowledgeable as Brock, but she still has a lot of battle experience.
Brock has the most battle experience overall, has travelled three full regions, is probably more knowledgable overall (about battles, medicine, and pokemon in general). His starter is also a Steelix that can mega evolve and tank fire-type Z-moves. He also branched out to train types other than his specialty (Ludicolo is basically the polar opposite to his specialty, he also has Chansey, Marshtomp and Forretress). So he's got the strongest pokemon, the most experience and knowledge, the best team diversity options and a mega evolution.
Until Iris got her offscreen level up Brock was definitely the strongest. Which fits since most feel he's also the best companion overall.
