On top of being a solid fire-type Pokemon, Arcanine just looks majestic as shit. Man's best friend? More like man's BEAST friend. His base stats are crazy-good too, if you're into that - but even if you're not, it's hard to deny that this fire-canine could light up anything it went up against. Also, its species is "Legendary Pokemon" - even though it's not an actual legendary. There are multiple Arcanines running around and ALL of them are "legendary." That's how badass it is.
If you can bear to trade away your beloved Haunter, something magical will happen - it turns into Gengar, the highest-ranking Ghost Pokemon of Generation 1. As creepy as a smiling gas-ball or licking ghost might be, Gengar stands above the others with his huge, toothy smile and the fact that it looks an AWFUL lot like the undead spirit of a Clefable. Seriously, compare Clefable and Gengar. Someone killed a bunch of Clefables and that's what placed number 6 on this list.
I'm not going to try to explain how a majestic blue eel turns into a big orange dragon, but Dragonite might be the most satisfying evolution in the game, if only because it seemingly takes forever to get to: the guy doesn't evolve until level 55, the highest level evolution in the game. It also helps that it ends up being one of the best Pokemon in the game, even if it does look like a jaundiced Barney the Dinosaur.
The most legendary of the legendaries, Mew existed only as an urban legend (there's that word again) for a long time. Rumors ran abound about how to catch it (including one involving pushing a truck that literally everyone in the world fell for) - and a way was eventually discovered of how to get it, although most settled for Game Genie/Game Shark cheats to get a team of Mews in the game. And the rarity came with a good reason - it was an extremely good Pokemon to have on your team: it could learn virtually any move. But perhaps the most impressive thing is that it never appears in the original PokeRap.