Easily Digimon. The advantage that Digimon have is that pretty much any low tier monster can just evolve its way up to deity-level monsters and can digivolve/de-digivolve at will between them.. Pokemon only has a handful of deity-levels, an okay group of strong ones and the other 90% are just filler garbage monsters that don't get particularly better even when they evolve because Game Freak has no grasp on the concept of power structure. Pikachu can only ever become Raichu and Raichu sucks and even Light Ball Pikachu is objectively trash. Agumon can at least go up to WarGreymon for respectable power or even further beyond with stuff like Omnimon.
Another factor that leads Digimon to win this one is how Digimon's main battle system usually works. Digimon, like pretty much any other monster collecting RPG that ISN'T Pokemon (or even plenty of RPGs in general) uses a 3v3 battle system. Because Pokemon is so dependant on a 1v1 battle system, the skillsets are designed mainly around that. As a result, powerful moves like Earthquake become far weaker because they end up hurting the Pokemon's own allies, so plenty of Ground Pokemon that would like that move for STAB end up being a hindrance to their own army. Also, stat buffing in Pokemon is super weak because it usually only ever affects the user and healing moves bar things like Heal Pulse are also super weak because they can only affect the user. Not to mention, Singles gimmicks like FEAR Rattata become flat out unusable in a Pokemon army because it just gets ganged up on. Digimon's battle system of 3v3 allows for far more variety of elemental skills to do huge spread damage to multiple targets WITHOUT harming the allies, stat buffs that affect both the users AND all the allies, healing skills that can affect multiple targets on their army for optimal longevity. Also doesn't help that Pokemon are limited to 4 skill slots and even then are limited further by movepool while Digimon gives you far more freedom to customize almost any build with 8 skill slots.
Pokemon loses this one hands down and way too easily.
