It was the most frightened I had been in my life.

Yes, it is acceptable. Whether it is the greatest English sentence ever composed is debatable.

I think you meant;

I was the most . . .

it is more natural to say " I had never been so frightened in my life"


Hamid

Thanks, GPY.

So all of these correct too?

What's the most frightened you've ever been in your life?
It was the most enjoyable/funny/relaxed/stressful I ever felt during my trip overseas.
It'll be the most stupid you'll feel after telling him the truth.

Tom

Mr. TomWhat's the most frightened you've ever been in your life?

OK.

Mr. TomIt was the most enjoyable/funny/relaxed/stressful I ever felt during my trip overseas.

"relaxed" works. "enjoyable" and "stressful" don't fit (you do not "feel enjoyable" or "feel stressful"; these words apply to the situation, not the person).

hrsaneiI think you meant; I was the most . .

It was the most frightened I had ever been in my life is possible. The 'it' refers to the (previously mentioned) experience of being frightened.