I'll list my top 5!
Where is Dragonite, you say? ._. That Pokemon is ugly as sin, and had no business or right to evolve from such a Pokemon as beautiful as Dragonair. If it had ANYTHING that resembled the beautiful Pokemon it was, instead of the ugly Barney "cutesy" dragon it is now, I'd forgive it.
Now Flygon, Altaria, Gible, even Girtatina-O and Dialga, THAT shows you adorable dragons done right. Dragonite, is both a failure and trying too hard. Ech.
...um, rant-bias aside, allow me to go to reasons.
Salamence, because he too is adorable, plus he got kicked into Uber, right? That shows he's toughsauce indeed. His cry is also love.
Garchomp is next because of how tough he looks. Land-shark, it took me a while to get that pun! I like Gibles, too, which is why Garchomp gets my next vote.
Tyranitar, mostly for the Celebi movie. ;A; That timid Tyranitar is the cutest giant-shy Pokemon ever! Until he got corrupted, that is, poor guy...
Metagross because the psychic/steel type is pure badass. Plus you got to train one thanks to Steve. 8D Hehe...
Finally, Sazandra. I'm sure he'll move up in line once I see him in action somewhere in my White version. He is quite cool, though. Dark/dragon! That is pure badass as well!