So what makes them (videos are below) work? As I see it, the key parts are:
1. the accuracy/vividness of the voices and mannerisms
2. the long build-up with minor gags and punchlines
3. a definite final punch-line
4. an original premise
I'm tempted to lump act-outs and impressions as the same joke, although I suppose the difference is an impression is usually a short joke with one target. Below is my single favorite act-out. It's the Stomach Bouncer bit from Jim Breuer. Wacky concept, nice build-up, good impressions, and a great closer.
Robin Williams on Golf, ditto.
Jim Carrey on Canada- this one is more of an extended impression, but I count it in.
And finally, a fine musical act-out from Kevin James, who left stand-up in order to appear in landmark comedies such as Paul Blart: Mall Cop and Zookeeper.