Now that I think about it, If I take elemental chips and EX skills into account, the Shield Boomerang (defense weapon in Zero1~3, uhr) would be the best shield weapon...but that seems a bit unfair, seeing as the EX skills are the "weapons" of the Zero series.
I think Leaf Shield, since it has a little more range at times.
Leaf shield does dick against Wily. as do all other weapons besides bubble lead (iirc).
Has there ever been a shield weapon that could be used on the final boss? I don't remember any besides Rolling Shield.
I honestly don't know, perhaps Star Crash (tho that should be empty from use against the Wily Press)
Also, using the charged Rolling Shield against Sigma is...unpractical? I have to try it, but you would have to activate it as you jump of Sigma's claw, since that cancels CRS, *shrugs* oh and then again, there is the shield boomerang, OH so useful against SeraphX on Hard, reasonably useful against Elpis and Omega...yeah Rolling Shield doesn't have squat on it.
-Elpis