It's been my experience to find the ones that have vertical legs instead of the angled legs. The ones with angled legs end up having a smaller portion of shade (usually more like 8x8 instead of 10X10). I had a killer one that had two end awnings that you could angle up to give more rain protection or drop to block the sun, but I lost it to the wind when I picked up the dent on my truck... One of my buddies had a killer canopy that the legs were made from 3 telescoping poles instead of the typical 2, which made it fold up really small and fit under the seat in his VW bus. I have never seen one for sale though. I should ask him where he got it.
I don't think that there are any really any that are better than the others. I would find one locally that you can look at and return without much hassle. The best thing to do is make weights that will keep in anchored. We put on a car show and, after a fly away tent caused us to buy a new paint job on a show car, our insurance company mandated that each leg have 40lbs of weight on it. I've made 4" PVC tubes filled with concrete for mine.
If you want one on the cheap, go to a trade show/camp out/large event after a storm and collect the pile of tents next to the trash and build you one good one out of the pile.
96 Rodeo Build thread
70 VW bus turned camper
86 VW Diesel Golf
07 Frontier V6 6 speed
1999 4Runner Limitee- wife's
2004 Genuine Stella Scooter