While my opinions won't be quite as in-depth with moves stats or abilities, I'll give it a go.
Gen 1: Charizard. Generic, I know, but I'm a sucker for fire breathing almost-dragons. And once it becomes mega charizard X it finally gets the dragon typing it deserves.
Gen 2: Feraligatr. I don't know why, but it reminds me of the crash bandicoot villain dingodile, and I love it.
Gen 3: Swampert. Specifically for the memories of when I first got Pokemon emerald and chose mudkip, got halfway through the game, and then my sister accidentally reset my game. Despite that detour, I started again and got to enjoy battling with swampert all over again. EARTHQUAKE TO VICTORY!
Gen 4: Torterra. RIP my first gen 4 starter in pearl, despite the massive weakness to ice, it was a powerhouse, and holds a special spot in my memory. If only my pearl game hadn't been stolen.
Gen 5: Emboar. A giant fire pig with fists? Why, yes please. The first Pokemon I ever got to level 100. Good memories.
Gen 6: Greninja. The thought of a ninja frog sounded silly until I saw Greninja. Who would have thought that something using its tongue as a scarf could pull it off while looking so AWESOME.
Gen 7: Decidueye. I was gonna say incineroar, but the name pun you can use on decidueye is just too glorious to put it in second place. Long live Robin Hoot.