Most has two closely-related meanings.
1 the largest in number or amount
2 more than half / almost all of somebody/something
As that OALD link says, the most is often used for the first meaning - but informally, the is often omitted. It's never used with the second meaning.
Suppose The Queen of Hearts were to ask "Who ate my tarts?"...
A1: "The knave ate the most"
A2: "The knave ate most"
If several courtiers ate one tart each, but the knave ate two, then either answer could validly be given, since he ate more than anyone else (but note that A1 is a "very slightly formal" usage).
But if the knave actually ate more than half (but not all) of the tarts, only A2 is correct. Though the Queen might not understand that intended distinction, since the could be omitted anyway.
It's worth making the point that the is never included when most simply means very...
"I am most grateful for your help"
...but interestingly, whereas in...
"There were 50 people there, at [the] most"
... the is normally omitted, it's almost always included if we add very as an intensifier...
"There were 50 people there, at the very most"
The Most Rude, ill-Mannered, and Humiliating Plays in NBA History!
If we use rudest here, we would have to still use "most" for the other adjectives:
The Rudest, Most ill-Mannered, and Most Humiliating Plays in NBA History!
The title uses "most" to apply to all three adjectives, because "ill-Manneredest" is a word I've never seen nor would wish to see in a title, unless written in jest, and there's no "humiliatingest".
Hence, the most logical, or logicalest, way is just to attach "most" at the start of the title.
Best Answer
Both are somewhat correct, but it's better to use simplest. This is called the superlative in grammar. It can be used by either adding the word "most" before the adjective, or by modifying the adjective with the suffix "-est". If you care about the rules, here is a quote from oxforddictionaries.com:
emphasis mine. Also, if you look it up in the dictionary (e.g. here), you will see "simple - simpler - simplest".
More than that, Simplest is also used more often that "most simple", you can see a clear tendency at this ngram (Sorry, it's small when added here as a picture. The blue line is "simplest", and the red line is "most simple"):
It is pronounced as
\ˈsim-p(ə-)ləst\, so you can pronounce it either with 3 syllables, or just 2 if you omit the firstə.