My personal preference is for Ambi-Pur. They come in some nice fragrances, and the vent mounting means that you don't have the problem of DSA tape, which either leaves a mark on the dash, or worse, comes off when the bottle is half full.
Dash mounted bottles also sometimes spill, and leave a mark on the plastic which is difficult to remove.
Additionally, my car always carries a bottle of room freshener for a quick solution, and the boot always has a bathroom deodorizer (Mogra!!) The deodorizer, in addition to the mild bouquet, also absorbs annoying smells in the car.