"Simple is best" vs "Simple is the best"

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