Which is the most difficult part in design a PA?

what type of PA?

feedforward double loop PAs are well-known and popular, but still hard to tune and get it right the first time..

digital predistortion is not easy also, since it is relatively new method..

In short, linearizing a PA is hard..

or maybe you meant just circuit level, like matching ...etc. that is relatively not that hard, just spend a couple of days twieeking ...etc.

I am not sure did I understand you correctly, or if I answered your question...

Cheers...

I think linearization is an important part of designing a PA. Generally speaking, linearizing method includes predistortion,negative feedback,feedforward. And predistortion can be designed at RF ciucuit and baseband stage. which one is more effective and easy to design?

Most effective: double loop feedforward PA, no doubt :)
It is very expensive, but very wideband, a lot of linearization (you can get >25dB correction easily) and well cosumented so should not be that hard to design (though you need a relatively big team and a lot of resources)...

Cheers...

"well cosumented"

I meant----> well documented