Of the D&D Chromatics, I'd have to say the Green. I just liked the idea of this huge creature stalking through some primeval forest. But I never liked the good-aligned Metallics, or the Gemstone varieties.
However in my own campaigns, my dragons to date have always been reddish-black, fire-breathing, very, very rare, and of any alignment... what I would call the classic dragon from literature.
In one homebrew, Silverdawn, however, which has colour as its theme, dragons are of the seven rainbow colours, any of which can be of any alignment...
Red = Fire breath
Orange = Acid breath
Yellow = Poison breath
Green = Gas breath
Blue = Cold breath
Indigo = Lightning breath
Violet = Steam breath
Black dragons are actually undead dragons (the colour drains from them when they become undead), and they breathe a magical black Disintegration cloud (the Shadow).
White (or Silver) dragons were the first dragons, and are now extinct (their breath weapon was a form of pure magical energy, the Radiance)... their offspring were the coloured dragons as detailed above.
The idea is working well so far.