Dear Mr. Smith is the best way to go. That is, Dear Mr. Last Name.
If you want to use "hello," which is less formal than "dear," you need a comma after the "hello." Hello, Mr. Smith. Why is this necessary? Because Mr. Smith is a vocative.
This video is great: https://www.youtube.com/watch?v=4_9_YSk-XGQ.
I like your name :p