To be grammatically correct, should the following sentence end with an exclamation or question mark?

Question

How nice it is !/?

Answer 1

Exclamation mark.

http://grammar.ccc.commnet.edu/grammar/marks/exclamation.htm

Answer 2

Thanks! Is the exclamation mark added even when considering only grammar and not emotions?

Answer 3

The reasons for using the exclamation mark are especially well stated in the linked webpage above.

You'll need to give me some examples for the rest of your question: "... even when considering only grammar..."

Answer 4

Well, suppose I say, "How nice it is?" Would that sentence be termed as grammatically incorrect?

Answer 5

Simply because the sentence begins with an interrogative word doesn't mean it's a question. A question mark there is an error.

Ask yourself ... is this truly a question? Or is it a remark about beautiful weather or a wonderful party? Or?

Grammatically, this is not a question. It's a remark that needs an exclamation mark at the end.