Stupid is as stupid does. While Spike makes some really poor and impulsive choices at times, I think he's quite intelligent for being able to keep up with Twilight's OCD and for playing the straight man when other ponies let their own fears and apprehensions get the better of them in what should be simple situations. At times he's the one losing it, but given that this flips back and forth depending on the nature of the situation, I think he's a well balanced individual. Not the sharpest tool in the shed, but by no means is he stupid and possesses a wisdom of his own.
A dragon's greed is natural, but given that he's demonstrated he can give freely of his possessions, time, and of his own life on several occasions, I don't think that's too bad either. Again, he can go back and forth on this, and seems balanced well enough to me. Besides, good intentions not always executed appropriately doesn't necessarily make one a bad person. I think that's what we have going on with Spike sometimes.
I think Spike is the most "human" of that lot. Although flawed, his flaws are often his greatest strengths under the appropriate circumstances.