Ad hominem fallacy
From Atheism United
Literally translated as 'to the man'. 'To the person' would be more accurate.
This fallacy mistakenly argues against the person, rather than the person's argument.
The fallacy is easily illustrated with a simplified form: "My opponent is X, therefore my opponent's argument is wrong."
Aside from the non sequitur fallacy, this is perhaps the most common fallacy.
