I've said this in some comments, but basically this is my take on the whole love square thing.

Marinette/Ladybug "loves" (I say "loves" because I personally think it is more of an infatuation/crush on) Adrien.
Adrien/Chat Noir loves Ladybug.

Now here is where it gets interesting...
I also think that Marinette/Ladybug LIKES Chat Noir, and Adrien/Chat Noir LIKES Marinette.

BUT the reason why they deny their LIKES feelings is because, at this point in time, they believe that the person they LIKE and the person they LOVE are different people. For example, Adrien/Chat Noir does LIKE Marinette, but he calls her a friend because he believes that to love Marinette would be to betray his LOVE for Ladybug.

Also an example of Marinette liking Chat Noir but LOVING Adrien is when she says "Maybe I'd see Chat Noir differently if there was no Adrien" (in the Webisode 'Chat Noir as seen by Marinette').

Also can I just say, just look at Adrien's face when Marinette kisses his cheek in the Heroes Day episode:

That is most certainly not the way I would look at "a friend" if they just kissed me.

In Summary...
Marinette/Ladybug "LOVES" Adrien and LIKES Chat Noir.
Adrien/Chat Noir LOVES Ladybug and LIKES Marinette.