Because I feel that I am going to say so I always use following line:
“I will say/suggest that you must not go there.”
Is above line correct? or, should I use following line:
“I say/suggest that you must not go there.”
But I think that this line has connotation that I say something regularly or frequently.
You usage of the phrases “I will say/suggest” or “I say/suggest” is called a hedge. It is an additional phrase that you use to somewhat soften the message.
In the following, the parentheses include an unspoken, implied message, that you may want to make.
Assuming you want to use the word “must”, the strongest message you could make would be
You must not go there. (This is a command!)
This may sound like a command, and often you don’t want to make that impression. Adding “I say” (or “I say that”) creates a slight “way out”.
I say you must not go there. (But someone else may have a different opinion)
This “I say” is idiomatic and does not mean you say it regularly (as simple present usually means).
Further “softening” can be achieved by using “I would say”.
I would say you must not go there. (I would, if you asked me, but maybe you don’t ask me. So I didn’t actually say that.)
Using “will” creates a different effect; it stresses the statement (usually with audible emphasis on “will”) when this is a part of a longer sentence.
Although I understand your desire, I will say you must not go there. (I don’t want to hurt your feelings but I really mean it)
Using “I suggest” instead of “I say” is an explicitly softer message. You can’t use “suggest” with “must” – it would be a contraindication. “Suggest” is followed by “that” and a statement in the present tense. As stated in Nick’s answer, it should be in the subjunctive mood, but in informal English a regular (indicative) statement is commonly used.
I suggest that you not go there. (subjunctive)
I suggest that you don’t go there. (indicative)