There isn't an answer to 'whats the best',thats entirely up to you. For me it's the spoon n1,for you it won't be if you dont want it loud.
Wouldnt expect much gains manifold-back either,the ek9's are tuned very close to the limit of power already
Why go 'branded' when you can go custom,
if you want somthing not too loud i woud go with a mugen twinloop
Also whats your price range.... If you have no price range id say the Toda/J's Racing exhaust would be 'the best' on the market or if you dont have a huge budget go custom 2.5"