The best are homemade by far. Not only can they be tweaked to work on a specific animal but the weight can also be adjusted to take full advantage of one's personal abilities.
A homemade broadhead is probably the best for rabbits but will likely fail on a crow if you hit wing feathers as these feathers do an incredible job of limiting penetration where broadheads are concerned. Crows are generally very alert and not an easy target to move in on. A dart made from a coat hanger or bicycle spoke will penetrate much better and is less likely to "wind plane" off course the way a lot of homemade broadheads would when shot at the longer distances the crows dictate.
Squirrels are a tough nut to crack and whether or not any dart will work on them depends greatly on the shooter and his knowledge of their behavior and his own abilities. I've never hunted squirrels with a blowgun but have heard of darts bouncing off of them. I've also seen one taken out with a soda straw shafted dart with a paper cone that had an X-acto broadhead mounted in a wooden insert. The hunter adjusted the weight to match his abilities and the dart hit fast and hard for a double lung punch.