Articuno - My fav since i used her to beat the crap out of the Elite four to get money in Red.
Rayquaza - My fav since it proved to be even more effective than Articuno at this task (no need for alternative Pokemon, Outrage decimates all)
Scizor - Scyther was a quick fav in the original (so cool), but once i saw the evolution in the second gen, my admiration was quickly redirected (how could he be even cooler?)
Magneton - Has been an effective teammate since Red, and since the intro of double types (along with Steel as a new type), it has been even more effective. Besides, it's a collection of magnets, with robot-eyes.
Aggron - Cool, though, reliable, and immensely flexible. I was able to replace Magneton (I beleive it wasn't readily available in Sapphire. Or not early enough, at least), by simply using a Thunderbolt TM.
Gallade - I have not actually played with this one yet, but if he plays anything like Blaziken (Which is what he seems to do, just replace Fire/Fighting with Psychic/Fighting), then he will be an effective team member. I tend to have find trouble finding a Psychic Pokemon i actually like the looks of and enjoy playing with, but Gallade seems to fix both those problems (He definitely looks cool).
It would be my purely "I like how they look and play" team, without thinking of how well they complement each other (Now that i look at them, they seem to cover each others weaknesses and provide enough firepower to be an effective team. Hmmm...I will look into this).