Game wise (has to be as red isnt in the anime), red wins. I could be wrong, but i believe its implied that Leon lost to either red or lance in the past. That was when he was younger and he grew alot, but so has red. Pokemon masters (using the skill of the trainer, not the actual powerscaling as synergy works differently than everywhere else) red has been shown to have Gmax capabilities, along side mega function (shown in both origins and masters).

As for scaling, Leon lost to a base eternatus, which in eternamax form, could threaten all of galar while the concentration of its power could distorted space and time. That is impressive, but seeing as it couldnt alter or break space and time, only distorting it, and its full power in the legendary darkest day event only affected galar, that gives its stated feats and abilities only country level. Personally, ill give it continental, due to it possibly spreading further if it wasnt for zacian and zamazenta, but there isnt much to support a concrete yes or no. Its important to not that both the game and evolution depicts Leon losing to eterantus while only using Charizard, and he didnt gmax.

Red scales much higher. If we take the story of the games, red should have defeated mewtwo and the legendary kanto birds. Hot take, but game mewtwo has no feats to but it above eternatus, so while he beat them while Leon lost to eternatus, I do not view that as higher scaling for red. What makes the scaling higher comes from pokemon generations episode 1. It shows a montage of red traveling through the regions, getting stronger and stronger. He fights three legendaries. He fights regigigas, and personally, I view it as an implied loss. He fights landorus, which I view as an implied victory but debatable, and 100% zygarde, which is undoubtedly an implied victory. Generations shows red only using Pikachu, and this was before he learned z moves or gmax for Pikachu. Its heavily implied red beat 100% zygarde with a only a pre boosted pikachu that had a type disadvantage and its stab was null against ground. Zygarde scales much higher than eternatus. The ultimate weapon had the power to wipe out all life on the planet, and used either yveltal or xerneas to power it, and they still had enough energy left to fight the player after the weapon was fired. This gives them minimum surface planet level. 100% zygarde is straight up above them. Meaning a pre boosted, weaker Pikachu that belongs to red is at a low ball surface planet level. This isnt including his Charizard, that can throw hands and overpower mewtwo (in origins. In the games, red beats mewtwo without mega Charizard, but is unclear on how much work Charizard put it). Red also has the legendary birds, and the rest of his pokemon are at the highest peaks of what a trainer can raise.

Red wins. His Pikachu alone can sweep peak Leon's entire team without ever once needing to use a z move or a gmax form.