I went on this search not too long ago and came up with this conclusion:
A grease rated "GC" is the right grease for wheel bearings ("LB" for chassis lube).
Lithium or lithium complex base is the most common, and is moderately waterproof. Aluminum complex is much more waterproof and most are compatible with lithium. Calcium sulfonate base grease is somewhat more water resistant than lithium and compatible with lithium complex (not plain lithium base). Compatibility is important to prevent problems when the grease is intentionally or inadvertently mixed.
I went for an aluminum complex base grease in synthetic. Be sure the grease seals are new and the spindle surface the seals rub on is not pitted. A dab of Permatex gasket sealer around the outside of the seal where it presses into the hub may help keep water out.
For the money and ease to get through online I ordering I went with the Amsoil Synthetic Water Resistant Grease: http://www.amsoil.com/storefront/gwr.aspx