CG that still holds up today? As in, looks impressive',' or 'could be mistaken for a movie that came out yesterday?'
Because probably the one I'd pick is the Stained Glass Knight from Young Sherlock Holmes (which was on last night, great film.) It looks incredible, and it was the first photorealistic CGI character. It is integrated perfectly into the scene, it looks like a stained glass figure coming to life, and it would be really hard to tell that it was CG and not a stop motion or practical effect.
Next up would be Donovan's Death in Indiana Jones and the Last Crusade. You know it's an effect (obviously) what you don't know is that it was the first all digital composite.
After that obviously come things like Jurassic Park and Terminator 2, but I think it's interesting to note that computer graphics animation has been going on in films since the 60s, and there are some examples that you wouldn't even know had been done with a computer, and that's the mark (for me) of a truly great effects shot.
Good FX are unnoticeable as such, thus good CGI should be unnoticeable as such. Alfred Hitchcock said he received one complaint from a fan about having noticed that one bird in The Birds was fake. In that particular scene alone there had been thousands of fake birds that hadn't been noticed. I watch Terminator 2 and I know the T-1000 is a computer model. I watch Indiana Jones and I didn't even know that Donovan's Death was CGI until I went and looked it up, I thought it was a practical effect like in Raiders of the Lost Ark.