I've never been a fan of shields that had many holes in them, as they don't seem like they'd be either sturdy or effective. As such, I've always been fond of the Skrall shield and Kopaka Nuva's shield, though between the two I prefer the former.
Yeah, I'll have to go with Skrall on this one. The worst though would have to be Mata Nui's quote-unquote "shield".













