All 1025 Pokémon, though not all their forms or variants since that'd take too long (Alcremie alone has 64!
) Last year I revisited HeartGold, White, Black 2, Y, Alpha Sapphire, and Ultra Sun to help me finish this dex.
Currently I'm working in Scarlet to collect all the shiny starter evolutions, at least only the ones catchable in outbreaks thanks to the DLC. Maybe if I finish this I'll try to go for the Gen 9 starters as well though I've never been a huge fan of hatching eggs over and over (Y was the only game it was okay in thanks to the coin trick even though it messed up my 3DS).