Y'see, the whole problem with this argument is that "best" isn't being solidly defined. Some people are arguing that other engines are better because they are more powerful and the end result can be of a better quality, while others are arguing that Source is better because it is simpler to use and can give a decent result easily.
And some of you are just ignorant, arrogant pricks. Guess who.
Source is simpler, and other engines are higher quality. Just remember that when you argue that an engine is "better" without explaining how, all you're doing is giving your opinion, not a definite ruling.
TL;DR: Shut up.