For me it probably is Pokemon since the none of the other series that I’ve played had as many games as Pokemon. I played: Red, Blue, Yellow, gold, silver Crystal, ruby, sapphire, Emerald, leaf green, fire red, heart gold, soul silver, platinum, diamond, black, violet, sword (wip). I also played: Pokemon Pinball, Puzzle League, Stadium, Arceus, TCG (for gameboy color), Pokemon Gale of Darkness.
Other series that I’ve played a lot of the games are: all of Metal Gear Solid (including Peacewalker, Portable Ops, Ground Zero) — except the first one is WIP (I watched my ex play it and Twin Snakes so I know what happens).
Final Fantasy: VIII, X, XII (WIP), XIII, XIII-2, XV. VII is WIP. Also played Brave Exvius and a little of War of the Visions, Record Keeper off and on until it was shut down.