There was a place that reviewed air fresheners a while back. I can't find the link. Pine Tree was of course the front runner but ultimately testing showed that it was only partially effective and really only best for Jeeps with the 258. New Car (spray, not the hanging kind) was deemed pretty good but not compatible with Jeep vinyl seats as it could cause premature wear on the vinyl surface, but this only seemed to be a problem with vinyl made before 1982. Red cherry also rated highly because it is compatible with all interiors, you can get it at Wal Mart, and it's good for at least 5,000 miles. The drawback is that you end up smelling like a stripper so married men could face uncomfortable questions from their wives. Some people use a combination of Red Cherry and Wild Coconut but this "red nuts" combination only lasts half as long. If is good for removing deposits, engine sludge, and jock itch though.
A lot of folks like the synthetic ones like Wild Vanilla and Cinnamon Toast. These are of course good for at least 10,000 miles but they tend to get faded while hanging from the rear view mirror in the sun so they look old even though they still have perfectly fine air freshening power. You'll pay at least 3x more for them, though, so you do the math. Personally I lilke a synthetic blend because I tend to buy into marketing fluff and am easily swayed by bold keywords and big advertising claims. If I can get a Wild Vanilla synthetic blend for $2.93, I'll run with it, even if it's not full synthetic.
"I give you a republic, if you can keep it." - Benjamin Franklin