I like the cork ones from Fel-Pro #VS50156. Spray permatex gasket tack on both sides. Worked great a bunch of times.
...The secret is in the prep work. Have to get the heads and valve cover flanges spotless. You can't have them too clean! Then assemble. Torque to GM specs 44 inch/lbs.
If they don't seal, either it's not clean enough or the valve cover flanges are warped(probably from someone tightening too much)
HTH, CaptRob