What are the best type of skewers to use for ground meat kebabs?
Easy!  You want fairly broad, flat skewers, which will prevent the meat from rotating around the axis, like these:



Quite appropriately, I've seen these referred to as "Persian-style" skewers.  Not having a decent Middle Eastern grocery nearby, I instead use flattened bamboo skewers that I can find at my local Asian market, which look similar but smaller, like so:



As with any other wooden skewer, you'll want to soak these first so that they don't burn.

The main thing is that the skewers be significantly flat.  Stainless steel versions like this which are quite thin look nice, but they don't grip any better than a round model does.

Alternatively, you can find double skewers which do pretty much the same thing, like these:



If, like me, you bought a massive pack of cheap, round bamboo skewers before you figured this out, you can approximate by using two of them running parallel.  This is basically the same concept as the double skewer without them actually being connected.  The only disadvantage here is that you can find up with twisting if you're not careful, and they're a bit more finicky to use.

It goes without saying that any of these options work very well with just about anything else that you care to skewer and toss on the grill.