DECLARE
    @A integer = 5,
    @B integer = 4,
    @C integer = 3,
    @D integer = 2,
    @E integer = 1;

SELECT
    Most =
        CASE
            WHEN @A >= ISNULL(@B, @A) AND @A >= ISNULL(@C, @A) AND @A >= ISNULL(@D, @A) AND @A >= ISNULL(@E, @A) THEN @A
            WHEN @B >= ISNULL(@C, @B) AND @B >= ISNULL(@D, @B) AND @B >= ISNULL(@E, @B) THEN @B
            WHEN @C >= ISNULL(@D, @C) AND @C >= ISNULL(@E, @C) THEN @C
            WHEN @D >= ISNULL(@E, @D) THEN @D
            ELSE @E
        END;