OmegaRuby or Alpha Sapphire, great story, decent postgame, national Dex, all megaevolutions, good OST. Sun/Moon are good as well. HeartGold/SoulSilver and BW/B2W2 Are PEAK

as the other guy said omega ruby is a great game, if u want smt retro fire red and leaf green are great

Heart Gold/Soul Silver were my first Pokemon games, so I'm biased towards those games. Ruby, Sapphire, and Emerald are fun, and the remakes are an absolute blast. Platinum is also fun. B/W are solid, tho I can't speak for B2/W2 since they are on my backlog. US/UM were fun too.

I can only seccond Jerry on this. Alpha Saphire is a perfect entrypoint to pokemon. Tho many people are reccomending white, so thats probably really good also.

BLACK 2 BLACK 2 BLACK 2 AHHHHHH

definetly x and y if your on 3ds. thats what got me to play official pokemon over pixelmon