As told by John McCain, Republican Senator from Arizona, current Republican Presidential Candidate, in 1998:
Q: Why is Chelsea Clinton so ugly?
A: Because Janet Reno is her dad.
The beauty of this joke is that it manages to offend just about everybody on some level. You really can't beat it for economy. Also, important to note that in 1998 Chelsea Clinton was 18 at best, so in no way fair game for this kind of abuse.