If you get a good job and you need to resign from current job then what should be reason mentioned in resignation application?

I would state true reason that I need to resign to accept another chance or challenge. 

You should remain egal to yourself , say the real reason politely 

Someone indicates  personal problem in resign letter. However, I am in favor of real problem in resign letter.

It is easier to be said than done. If you are unable to mention on the resignation letter, you can highlight the exact reason on the 'exit form' provided later by the HR which will be treated as confidential by any professional organisation. 

I got new job that suites my future career path or expectations

Be clear and state the reason of leaving that now you have better opportunity. Resign with dignity don't be negative or un-professional when resign from your position.

We must mention real reason.

Burning bridges can often start uncontrollable fires.  If you cannot tactfully outline issues that led to you seeking a new postion, then it's best not to speak at all.  In that event, it is best to state that you are taking on a new challenge.

Well it can get tricky here on, you should mention the real reason but also keeping in mind that you shouldn't get carried away with too much truth like  'I can't stand my manager face anymore'. Yes! you are excited about your new role but flaunting that doesn't really show you in good light. 'I would like to resign because i feel I should explore different objectives and craft different goals'. Many managers understand the wits of a new opportunity and seldom you find a no acceptance unless you are contractually bonded.

The best option is to say the truth , never try to lie, because it will damage your relation and respect. Keep in mind never destroy your back bridges. True reasons and your honesty will convince your office