they are all better than the factory one. one or another might make 1-2 hp more than the other one. it just depends on what you want. full bolt on car, I would go with (trying to be impartial here) the courtesy 2.5 inch cat back. I got it on my car (muffler only though). stillen is alright, I had it on my car for about 1.5 years and no real complaints, not as loud as my courtesy, but droned alot. the hks I have never heard, the greddy sounds like a pissed off bumble bee when you are on it, but quiet at idle. real nice, and would be my second choice behind the courtesy. and then you could go into the custom kits w/ universal mufflers (seriously) like the stromung or the remus (no cheap 89 buck mufflers on your se-r please) sound real good and deep too.