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.
"Simple is the best" implies that simple is a noun. The meaning of simple as a noun is something simple, unmixed or compounded, a simple *thing* or worse, an ignorant, foolish or gullible person, a commoner, a "simple".

"Simple is best" implies that "simple" is an adjective, like "best." This is by far the more elegant way of expressing the concept because both sides of the "is" are the same parts of speech and express clearly the sense of [Occam's Razor](https://en.wikipedia.org/wiki/Occam%27s_razor) within this sentence.