Gen 2 was the one that added the most features. Day and night cicles, day care, breeding and IVs, a second whole new area to explore - kanto , etc. But if you wanna play it nowadays id go for heart gold or soul silver as they are better, updated versions of these.
If not, black and white alongside black and white 2, are among the best pokemon games, even tho they lack on pokemon numbers.
If you wanna go for the 3ds titles, all of them are decent. XY its cool but it lacks on depth and its way too easy, like really too damn easy; omega ruby and alpha saphire are the best on the 3ds - very pretty city design, you can fly on latios, get most of the legendaries, there are secret bases...there is really a lot of content! -, but again, they are too easy; ultra moon and ultra sun are refreshing, they switch the gym formula and there are trials instead, I like them a lot and the games are actually very challenging, the IA its actually decent and you will most likely die and game over some times.
As for sword and shield...dont get them. They re half assed pokemon titles which are absolutely not worth 60 bucks, they have less content than the 3ds ones, graphic wise they are pretty similar, they re even easier than xy - exp share its always on, there are tons of exp candies...you will be heavily overleveled just by doing the regular battles and even avoiding all the random encounters - and the story its literally the worst ive ever seen on a pokemon title