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Venusaur, Dunsparce and Starmie are my definite top three (in no specific order).
I've always loved Venusaur's design, it was my first starter and I remember being particularly fond of the art on the base set Venusaur card as a kid. I can't emphasize enough how magnificent I think its design is.
I fell in love with Dunsparce the moment I encountered one in Gold after hours of searching for it with no idea what to expect (I just went off something some NPC said about a rare Pokémon in the Dark Cave). It's too adorable not to love.
As a kid, it was mindblowing when I finally figured out how to evolve my Staryu in the high 70s and I've just really liked Starmie since. It also smashed the Battle Frontier in Emerald for me and the same Starmie was a staple on my Wi-fi teams in gen 4 before people expected Choice Scarf on fast sweepers.
Stunfisk (along with Crustle, which deserves an honorable mention) is one of the few gen 5 designs I liked the moment I saw it. Also a staple of my gen 5 NU team, which was the only metagame from that generation I could stomach. Kind of reminds me of Dunsparce.
Metagross is just a badass design and I enjoyed running some more creative sets on it in gen 4. It really did some work for me back in the day.
Forretress is just another great design and my goto spinner in gen 3 and 4. Zap Cannon was tons of fun against setup sweepers despite being unreliable.