Allegedley the ITG gives most power and the noise along with it, Twintake not far behind, then its Evoms, Dbilas, AWE or Volant (AWE and Volant are both for K03 but a simple drill tap and add feed tube means they fit the K04 just fine.
Twintake, Dbilas, Evoms and ITG/VWR are all around £350-360, AWE is £340 from UK supplier or if you have a relative in USA to post it on you can prob get the AWE for around £290 and Volant is £167 plus shipping (again from USA) so more like £220.
Depends how much you want to spend, noise levels and ease of fitment will be defining factors on your choice.
Personally I prefer the closed Box design of AWE as its a closed carbon box and should be a little quieter than most plus it looks more OEM in my mind (especcially if you adapt your engine cover or buy a replacement like Evoms engine cover or Neuspeed etc.
Most on here will point you towards Either twintake or ITG/VWR, same as remaps its usually Revo or APR and exhausts is usually Milltek