Are we talking about games or just pokemon? Well I consider both.
Gen 7 has my favorite single player campaign for sure, and I love alola, plus it easily has the best cast of characters and a fun story. My only complaint is that the amount of new pokemon is kind of small but it unlike gen 6 they are overall a really interesting/fun set of mons and feel very fleshed out, actually gen 7 did wonders in fleshing out pokemon from a worldbuilding perspective and introduced some really interesting things. I like how gen 7 embraced pokemon being a JRPG more than previous ones, though I know some people were super bugged by cutscenes and such but I found myself so much more engrossed with the journey which I haven't done since BW1, probably the only pokemon game whose narrative even comes close.. I do kind of wish the outfits had more variety and alola a bit bigger but gen 7 surprised me in how much I found myself enjoying it. And that music..yeah it and gen 5 run really close.
Gen 5 has my favorite generation of pokemon as it offered the most new pokemon and Unova really felt like a place of its own as a result. BW1 was the first pokemon game where the story felt like something worth being invested in and not an afterthought, as well as having more meat to its cast compared to previous gens. It also to this day has the best rival, N. Unova is my favorite region and the music for gen 5 is fantastic. PWT is also one of the best battle facilities ever, with emerald's battle frontier being the only thing that comes close.
gen 4 is what got me back into pokemon so it holds a special place in my heart, plus by adding online and the phys/special split it dramatically changed the gameplay for the better. While I wasn't totally sold on the pokemon themselves initially, over time it became one of mu favorites and now is probably one of my favorite generations pokemon wise. That said, while I know it's really popular, I kind of find HG/SS not all that great despite G/S/C being my favorite pokemon games for years.
gen 3...ahhh okay. Sooo like I have mixed feelings on this one. On one hand, Emerald version is iMO one of the best pokemon games and probably the best -pre-DS pokemon game. The pokemon are also quite exotic looking and while it was my favorite generation of pokemon for a while they've been surpassed by later gens I feel. Also I feel FR/LG are some of the dullest main games in the franchise and it was this gen when I feel out of love with pokemon. In fact it was 3rd gen when pokemon's popularity seem to have a sharp decline. I didn't discover emerald until after 4th gen.
gen 2 was my favorite when I was little and really refined the gameplay, but I've realized that in gameplay these games have been surpassed by later gens and pokemon wise it's solid, but I now find myself preferring gen 1's set of pokemon over it.
gen 1 was good for its time, but I'm just tired of it, and how much it's been shoved down our throats over the years. The games are still a fun little novelty now due to how different they play, and I still like the set of gym leaders.
gen 6 refined the gameplay and gave us the lovely fairy type, and the best online. X/Y was sound mechanically and introduced some great ideas, but the cast was some of the worst in pokemon yet, the new pokemon were just too few and Kalos is easily one of the least developed regions yet. A France based region sounds amazing on paper but the execution left much to be desired, Omega ruby/alpha sapphire is probably the only remake that I can't say is the best game of that region gameplay wise as I feel emerald's postgame is more interesting(unless you really like catching legendaries) and its actual campaign is more fun since OR/AS is so mind numbingly easy as to be borderline boring. I do love hoenn but overall gen 6's lineups of games are the ones I feel least inclined to go back to.