Question: As a foreigner, using articles is one of the most difficult part. 

What is the difference between "Simple is best" and "Simple is the best"? When I googled it, both appeared with many results, so it seems both are right. Are they same or have some differences?

Answer: 
Logically they are the same, but they have different psychological connotations. "Simple is best" is a bit warmer and implies that the receiver is being edified. "Simple is the best" is stated in a way that makes it sound more like it's an objective fact or something. It's kind of hard to describe.

**Edit:** After nearly ten years of considering this every time someone upvotes this, I think the difference boils down to sexuality. "Simple is best" is feminine, warm, nurturing, cunning. "Simple is the best" is masculine, cold, challenging, confrontational.
The long and short of *this particular* phrase, is they're both the same in meaning, and they're both grammatical. Note that I have italicized "this particular". This is to indicate that "the" can make a difference in certain other situations, but not in *this particular situation.*

Both of them are stating that the speaker supports simplicity