assuming we're taking relative size into account and ignoring "well obviously a big pokemon like wailord would cum more lol", as well as ignoring any magical bullshit pokemon has to offer such as "water type = more fluids = more cum" (though water types will still dominate regardless)
the more common a pokemon it is, the more its species has to be breeding in order to make so many babies and thus there's a high demand of sperm to impregnate females + a healthy amount of competition forcing males to produce more in order to be the one succeeding
so very common mons like Magikarp, Wingull or your regional Rattata (did you know real life rats have gigantic balls for this very reason?) likely produce more than average
pokemon belonging to common types like water and normal, like in the above examples, are also probable candidates
field egg group being so large means a lot of competition, so anything in the field egg group also gets a boost
lastly we have gender ratios such as 87.5% male meaning more competition amongst males due to them greatly outnumbering females
given all of this I'd say the strongest contenders are
-for their incredibly common status
>Rattata and Raticate
>Zigzagoon and Linoone
>Wingull and Pelipper
>Psyduck and Golduck (in every region)
-for being water/normal types in the field egg group with 87.5% ratios
>Eevee and Vaporeon
>Oshawott, Dewott and Samurott
>Panpour and Simipour
>Piplup, Prinplup and Empoleon
>Popplio, Brionne and Primarina
>Sobble, Drizzile and Inteleon
femininity aside, I'd have to go with Primarina because mermaids are already a highly sexual concept and that little trap is gonna have to pump out some ridiculous numbers to uphold that
