They're mindless, which means they lose all of their feats from normal levels/HD. They do keep bonus feats, though.
- Consider Lantern Archons. Lots and lots of Lantern Archons. Swarms of them, really. They keep everything that makes them special as dustform (flight, ranged natural touch attacks that ignore DR, tons of defenses), and they only eat 1 HD apiece from your control pool. Plus, you get to thumb your nose at those wussy necromancers who can't even animate a ball of sentient light.
- Glabreezu are also really good. Noticeably better than a Stone Giant, and at 2 HD lower. Certainly good enough to solo most every encounter your character is likely to face at 5th level (barring a stupidly rare crit + failed save from a bludgeoning weapon).
- For 1 more HD, try a Horned Devil. Another natural attack, another manufactured weapon attack (at -10), spiked chain proficiency for greater reach, flight, +13 AC, +8 attack bonus, a whole lot of defenses, and telepathy because why not. Regeneration gets lost due to having no Con score, unfortunately.
Basically, what I'm saying is look at outsiders (and there are probably better options, as I've only checked up to MM page 60). They're really, really good for their HD, which is what you care about most when dustforming things (value per HD).