Although he remains as the main hero in all of Fate/stay night scenarios, the directions he takes are slightly different in each. In Fate, he decides to make peace with his past, and the story ends with him being more idealistic than ever and falls in love with Saber. In UBW, although he still pursues his ideal despite having to face its faults, he finds a small relief in his love with Rin. In Heaven's Feel, Shiro in the end abandons his ideal, exchanging it for the happiness of Sakura. In each scenario, he represents three different types and stages of a man's life: as a boy who dreams of an ideal, as a teen who wants to have both the ideal and the reality, and as an adult who accepts the impossibility of the ideal and compromises with reality.
Pick which ending makes you the happiest.