I had a bit of trouble deciding between the first five options, honestly haha.
In the end though, I decided I couldn't go past Caterpie/Metapod/Butterfree, and here's why. My first game was Red Version, where the Viridian Forest had Weedles coming out of every orifice in your body but there was hardly a Caterpie in sight. So I wandered around Viridian Forest for what seemed like hours, trying to find both a Pikachu and a Caterpie since I wanted them both for my adventure.
Then when I finally came across a Caterpie, it was the best feeling ever and ever since then I've always associated those feelings with Caterpie. Also, I love Butterfree's powers much more than Beedrill's, I find they came in handy much more throughout the game.