Nixon is "the worst" because he was the one forced from office. But he wasn't actually removed through legal process. They started the process, but he left on his own before it came to anything. Clinton was actually impeached, but did not leave. I dont think the US has ever removed a president legally. They have quit, which isn't the same thing. Korea is ahead in that regard. Im not sure how/if the american system might handle a legal forced removal.

In nixon's defense: read his wikipedia article. He wasnt a monster. Many good things came from his presidency.