I think some of the worst CGI I've ever seen was X-Men: Origins Wolverine. He had shitty looking CGI claws, some of the backgrounds looked horrible, and some of it looked like PS2 graphics.
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Yonggary. Haven't heard of it? Well you have now, and if you ever see it you'll wish you hadn't. Not only does it have some shitty CGI, it also has the worst acting I've ever seen, and that's saying a hell of a lot.