Gen VII: tbh, I didn't have a favorite generation until Gen 7. I love all the new Pokemon, the characters, PokeRide got rid of HM (hopefully forever), the story leagues better than X/Y, Team Skull in general, AND DAT MUSIC! My favorite OST in the Pokemon series. Also some challenging moments in the game (Not Reborn difficult, but still). Some downsides are the removal of Super Training and SOS Battle make grinding difficult at times, but hey every gen has it's flaws. I love Gen VII.
Gen IV=V: I know this is cheating but really like both Gen 4 and 5 equally for different reasons. Gen 4 had some solid Pokemon designs, memorable tracks, pokemon would follow you (HGSS), and the fact that HGSS exist. Oh yeah, and the fact I have a love/hate relationship with Cynthia. While gen 5 doesn't have quite as many good designs, It makes up for it with a good story, memorable characters, and I like our villainous team, Team Plasma. However, Gen 4's overall slowness REALLY bothered me and I didn't like the "weather wars" and the competitive sence in Gen 5.
Gen III: This gen made me a Pokemon fan and cared about Pokemon now. I love the starters (the best imo is Mudkip), love the tropical setting of Hoenn, pretty good pokemon designs, and (I can't believe I'm gonna say this) I love the Pokemon contest. But, the encounter rate was a bit much (mainly in the surfing segment) and I didn't care much for it's story. Plus, ORAS fixed everything I dislike in Gen 3.
Gen II: I really like Gen 2 for nostalgia. Awesome Pokemon designs, the best post-game to date, and the final battle with Red was the icing on the cake. Unfortunately, Not a lot of merorable characters in my option, didn't like the rival, and I like HGSS better (that's a petty reason, I'm sorry :<).
Gen I: I did like all the Pokemon, Team Rocket, and the twist at the end where the champion is our rival. However, I find Kanto very boring to travel thru and it's very outdated in mechanics. It may have been awesome in the 90's, but it hasn't aged well
Gen VI: What. A. Mess. I enjoyed the Mega Evolutions, the inclusion of Fairy type, and the jump to 3D. The story was a mess, I didn't care at all about the characters, Team Flare was the worst of the villainous teams, and even without the EXP Share on the game was WAY TOO EASY. Gen 6 only saving grace was ORAS. But really, Gen 6 had it's good moments, but overall it was a mess...