It's the first legendary Pokemon you encounter in the games, yet that wasn't quite enough to crack the top ten: Articuno was one of the few Ice-type Pokemon you can find in the first generation, and probably the best. According to its R/B Pokedex entry, it appeared to doomed people lost on icy mountains - which begs the conspiracy theory: "Was everything after your meeting with Articuno the fever dreams of a dying child?" Answer: NOT ENTIRELY IMPLAUSIBLE.
Leading off the top ten comes one of the coolest-looking (if not performing) Pokemon: the badass bladed bug Scyther. In later generations, he would evolve into the similarly badass Scizor. But really, if you're dead-set on using an oversized bug on your team instead of dragons and ghosts and psychic cats, you could do a lot worse than Scyther.
The only Pokemon to be defined by his facial hair and predilection for holding kitchenware, Alakazam was the only three-stage psychic-type evolution back in the days of Red and Blue. Known for its powerful psychic moves and the cause of one of the most hilarious lawsuits Nintendo ever had to put up with (well, technically that was Kadabra, but c'mon), Alakazam also finished off the best naming-scheme of any evolutionary chain in the series (Abra - Kadabra - Alakazam!). Also, it's super-effective against Vanilluxes.
One of the most shocking and satisfying experiences of the original Pokemon games were evolving your stupid, splashing, worthless Magikarp into a huge terrifying dragon. Having to start all your matches with the pathetic fish, pull it out and finish off your enemies so it gets a little EXP was a chore - but it all paid off with a Hyper Beam-blasting Gyarados. Thank Arceus for Poke-puberty.