The only ones with a potential to generate power are the 3.5" ones that have a larger MAF housing, also requires a 75mm throttle body and a tune. Will not run right out of the box.

The rest just make a pretty sound, some of them actually cause a loss of WHP, there's literally hundreds of threads on the subject for you to browse.

 

IIRC, the Stillen/RC2 ones are most popular, not because the produce more power than stock but because they aren't as affected by heat soak as all the other open set-ups.
That being said, I noticed a slightly harder top-end pull when I went to a Stillen kit, as well as an amazing induction sound. Would probably prefer RC2 due to the sealed design, but it's impossible to find nowadays.

 

Have you done any research? The information is out there.

 

Lots of info on this subject can be found in this forum. No need to clutter it up with another thread so I'm closing this thread, OP head over to the intake/exhaust sub-forum and start searching/reading. You'll also find out that unless you're doing a full build on your motor headers aren't worth the expense and pain in the azz to install. The power gains are disappointing.