Purely based on Tennis ability and talent, Simon by far, the most underrated guy in tennis especially on these forums. Almagro second.
These 2 are hugely underrated by the kind of frustrated small boys who post most vocally here but all tennis players take them pretty seriously it seems to me.
Simon has a lot more pop on his shots when he wants to than people realize. His acceleration is actually pretty impressive when he wants it to be and Almagro if he had a little better head and movement could really be competing for a lot more.