Expanding on Xin's answerXin's answer and assuming the comparison value type is INT, this approach works too:
SELECT IIF(ISNULL(@A, -2147483648) > ISNULL(@B, -2147483648), @A, @B)
This is a full test with example values:
DECLARE @A AS INT
DECLARE @B AS INT
SELECT @A = 2, @B = 1
SELECT IIF(ISNULL(@A, -2147483648) > ISNULL(@B, -2147483648), @A, @B)
-- 2
SELECT @A = 2, @B = 3
SELECT IIF(ISNULL(@A, -2147483648) > ISNULL(@B, -2147483648), @A, @B)
-- 3
SELECT @A = 2, @B = NULL
SELECT IIF(ISNULL(@A, -2147483648) > ISNULL(@B, -2147483648), @A, @B)
-- 2
SELECT @A = NULL, @B = 1
SELECT IIF(ISNULL(@A, -2147483648) > ISNULL(@B, -2147483648), @A, @B)
-- 1