Opinions will vary however I'd recommend one of the Audiophile-type outlets.
After trial and error, I settled on the Furutech GTX-D Rhodium outlets. I'm not stating it is the best but it's what sounded the best to me.
Just remember that the Furutech GTX-D Rhodium outlet has about 300 hours of break-in to achieve what will become its signature sound. I realize some will criticize me for what I just stated but if you research the outlet you will see that many others agree. One should not be critical unless they have heard and experienced the product in question.