To a friend or a kid, you can say 너는 항상 옳아. (옳다 = right) 너는 항상 맞아 is okay, but it is more like "You are always correct". Also, 맞다 has many meanings. It can be interpreted as "You always take a (physical) beating" for example. To adults, you can say, "XX 씨는 항상 옳아요", or "XX 씨는 언제나 맞아요", where XX is the person's first name. 항상, 언제나, and 늘 all mean "always".