While de-energizeing the line feeder and after opening the CB from both sides, what is the right to open the BB isolator OR line isolator first and why ?

The two Circuit breakers, are they at opposite ends of a line, or are they at opposite ends of a fixed bus?

Generally, I would prefer to work at one end of a high voltage line at a time, since this will reduce the need for communication and make it easier and more secure.
With this in mind, I would:
1: Open Circuit Breaker.
2: Open the associated Line Isolator.
3: Apply earth between Circuit Breaker and Line Isolator.
4: Open the Busbar Isolator, and earth between this isolator and Circuit Breaker.
It is now safe to work on the Circuit Breaker.

If work is required on the interconnecting lines between two switch yards, I would then carry out the same sequence on the relevant Circuit Breaker at the other end of the line.

Finally, I would then apply earth at both ends of the line, as well as within view from the relevant work site. (After testing for voltage first, of course).

Mr. Adel Elsayed, It is difficult to answer your question unless you please further clarify the reason for your asking the question. However, if the line feeder is radial (which it would be since you have called it a Feeder) then it would be better to also open BB Isolator so that an accidental Closing of the CB doesn't cause unwanted and unintended Energization of the Line.
However, it should just not matter whether after opening the CB feeding the Line from BB, the BB Isolator is opened first or the Line Isolator is opened first, should not matter, if I have understood your question correctly.
Regards.

Dear all, the feeder is connected in ring network (400KV) and the background of my question Suppose i'm opened the CB's from both sides and due to the gadding capacitance across the CB and the generated capacitance across the OHL (over head line) the line VT (voltage transformer) is damaged as a result of ferroresonce phenomena.

I will be surprised if your line VT can not be earthed as soon as you have opened the nearest CB and Line Isolator. Then there should be minimal risk of damaging it through ferroresonance.

Mr. Adel Elsayed, After getting to know that you are faced with a 400 kV ring-main connected network having grading capacitance across CB, I strongly feel that the matter needs to be properly analysed and Switching Guidelines decided accordingly. Else, there can be serious chances of ferroresonce damages.
Therefore, I would not like to offer offhand comments on this serious matter. It is advisable to consult a circuit analyst in this matter.