Simple Green or LA's Awsome(Dollar store) are very good degreasers.

If your valve cover is aluminum and has any corrosion or oxidation a very good product if you can source it, is Flitz Industrial Strength Aluminum Preclean. About $15 for a 16 oz. spray bottle but it does wonders for things like intake manifolds, valve covers and other cast aluminum engine parts. It leaves a protectant film as well.