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

Use "Hello, Mr. Smith." We always use the surname with Mr/Mrs/Ms/Miss, not the first name. "Hello, Mr. James" makes me think that James is the surname. "Hello, Mr. James Smith" is awkward and overly-formal. You'll never use this phrase.

Mr. Smith or Mr.James Smith is correct, because this is formal way of talking. If you want to call him/her by first name, use Hello, or Hi, Hey....instead!

Hello,Mr.Smith

I think "Hello Mr. James Smith"