Gen 1; Porygon: Love this thing. I liked the idea from the first time I talked to that dude in the Mt. Moon Pokemon Center that there were Pokemon you had to get in ways other than catching them, & Porygon epitamized that with its price tag of 200,000. On top of that, its design is cool, & its Conversion attack is useful.
Chansey: It feels like a prestige to have one.
Farfetch'd: It's a bird that carries a leek. It's design is so interesting, & with Swords Dance & Slash it's not bad in battle either.
Alakazam: My best friend's favourite, back when he played the game. I didn't like it originally, but it sort of grew on me...
Gen 2; Porygon2: (see above)
Ampharos: In my mind, this guy's as boss as Alakazam & Dragonite. Thunderpuch ftw.
Jumpluff: ........ It's cute. And it's every attack is a status ailment move.
Smeargle: Sketch. SO much fun, being able to have a Pokemon with nearly any moveset in the game.
& Blissey, because it feels empowering to look down & see that 711 HP