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: 
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
"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.