Have only tried a couple brands of cords (Audioquest, Belden home brew, and Carol cable home brew). All three were better than stock (they were 14 - 12 ga. each and shielded - the home brews with aluminum foil!).
However, nothing prepared me for the benefit I got with LAT International's AC-2 cord (on the web at www.LATInternational.com). It's amazing, sincerely - I now own three. At $129.00 per, they are a very effective upgrade.
Enjoy!