For the main-line games, it depends what system you have, and if you have/had a DS/3DS/Switch before it should've been hard to avoid them..
Anyway, for
DS go with either
Platinum or
HG/SS (HG/SS are the same game, with just some different exclusive Pokémon you can catch in game, though you can trade for them so no worries..),
Platinum is a Gen 4 game, HG/SS is Gen 2 Remake with some Gen 4 elements & Pokémon..
There's also Gen 5, that's
Black/White &
Black2/White2, is set in different region & with different Pokémon to catch..
For
3DS, for Gen 6 there's
XY and
ORAS, while ORAS is a Remake of Gen 3 with again some Gen 6's elements.. I would choose it out of the 2 games..
Gen 7, is
Sun/Moon &
Ultra Sun/Ultra Moon, they've changed a few things, no more Gyms etc. Though it's not a bad game, i would go with US/UM if you choose to go with this region first..
For
Switch games, there's
Let's Go Pikachu /Eevee, again same spiel with minor changes , and it is a Remake of the Gen 1 games..
There's
Sword & Shield (Gen 8), it's decent, though there's no national Dex..
There's also the Gen 4 Remakes
Brilliant Diamond /Shining Pearl, again same games as D/P & without the enhancements of Platinum...
There's
Legends Arceus but it's not a typical Pokémon game, i wouldn't suggest it to you for now, not it because it's bad but it's because it's different from the other mainline games..
I skipped GB/GBC/GBA games because i didn't you'd start with those.. Of all the above go with whatever you suits you and with whatever system you have, though if you have a 3DS you can play both all the DS & 3DS Games on it, in addition to all the GB/GBC/GBA Pokémon games..