If multiple potential love interests are available to the MC, the choice is extremely simple, ladies and gentlemen: 

Pick the person who makes the MC grow the most. Pick the person who grows the most because of the MC.

Pick the person who makes the MC question everything--their morals, their ideals, their goals. Everything. 

To see this in action, watch the video in the sidebar. Best. Love-scene. Dialogue. Ever.

Oh, and the above bolded statements don't just apply to love triangles. They apply to all romance. Both lovers in question should grow because of each other. That is the most satisfying romance, period.