Not gonna lie, I far prefer gen 6 to everything else because they have great ingame ladders. Ignoring that, HGSS and Emerald are my favorite games to play ingame, because I found Battle Frontier to be extremely addictive post-game content, and I really wish they'd bring it back for Pokemon Sun and Moon! I don't mind replaying the old games every once and a while for nostalgia, but fact is, the heart and soul of mainline Pokemon to me is teambuilding and battling, so just crushing the Elite 4 doesn't really leave me that satisfied!
Spinoffs however, are a very different story for me. I've found that most of my favorite Pokemon spinoffs tend to predate Gen 4. Like most people, I thought Pokemon Snap was a brilliant spinoff that made great use of Pokemon as wildlife, and I really wish they'd put it in the Wii U Virtual Console in North America! Pokemon Puzzle League is basically Tetris Attack 2, and for those of you who have never played it: I'd give it a try, since its one of the best puzzle games around on both the single player and mutliplayer front! The Pokemon Pinball games also rank among my favorite portable pinball games to play, and I still play them from time to time while on the go.
I'm even starting to enjoy Pokemon Colosseum again. Since I'm starting to doubt that we're gonna get Pokemon Gold and Silver for VC in time for Pokemon Sun and Moon's release, I've decided to give that game another go so I can complete the Johto Pokemon in my living Pokedex, and I guess I'm really enjoying that all the battles are doubles unlike the last time I played it (which was probably about a decade ago now lol). I even sorta like Hey You Pikachu, which is one of the few games I'd call "so bad its good" (in fact I'm damn near tempted to make a thread about that game lol).
Meanwhile, post 4th gen, I don't find nearly as many spinoffs that I really like. I will admit that Pokemon Battle Revolution holds a lot of dear memories for me, since its random match up was my first real exposure to online Pokemon battles, and its also how I met
Jibaku and got into Smogon eventually! However, outside of the first taste of online battling, PBR was pretty dreadful for everything else, and paled in comparison to the other console Pokemon games from previous gens content wise. Pokemon Rumble World got me hooked for a bit, but I felt that it was too simple and mindless to really spend a lot of money on, so I can't imagine I would have been satisfied with playing the non F2P ones. Also, I never really got into the Pokemon Mystery Dungeon games, which I know a LOT of people enjoy, so maybe that might have a lot to do with my general apathy when it comes to newer Pokemon spinoffs.
That being said, I found Pokken to be a surprise hit for me this year, so maybe TPC and Nintendo are starting to get smarter when it comes to Pokemon spinoffs. I feel like most recent Pokemon spinoffs seem to feel... sorta like cheap cash-ins, but Pokken shows that when effort is put into them, they can still be great! And like I said, I never really got into the PMD games, so maybe I should give the new one a shot later...