I've personally found:
All - 2x manufacturer's warranty, up to 1 year additional, on top of:
MC - 1 year original warranty
Visa Plat/Signature - 3 years
AMEX - 5 years
Citi - 1 year, regardless of card network
I heard that Visa includes remanufactured/refurbished items while everyone else does not, but I didn't verify that.
In any case, I try to put my important items on AMEX or Visa cards, and stay away from Citi and MC if warranty is a concern.