The first time I played through, I'm pretty sure I just kept trying with Pikachu over and over (not even bothering to level up in between fights--Yellow was my first game) until I got frustrated and bugged my parents to let me use the internet to figure out what to do (back when we had Dial-up, too, so I couldn't just go on whenever I wanted). I then used Butterfree pretty much every time (and usually by the time I figured out Butterfree sucked, I was too attached to the thing to let it go--I believe I still had it with me when I faced the Elite 4 in several games). I think I've used Nidoran a couple of times (now that would be my Pokémon of choice, though), and I don't think I've ever used Mankey.