What should i use S21 when matched to 50Ohms or pure S21 when choosing FET?

I am a little comfused about choosing best suitable transistor for certain S21 magnitude. What is the fast way of doing that? For now, i use s-parameters, then do simple match of gate to 50Ohms and see if s21 magnitude is enough. But maybe there is some other faster way, or rule. So i can just see at bare S-parameters in datasheet?

I am afraid you have to design matching network for every transistor. I will recommend to stick with your S21 as it is crucial. Moreover transistors with S11 or S22 (that u can see from its list) close to 50+0j at your desired frequency band will obviously need much simple matching network compared to the one with impedance greater than 50+0j. so the bottom line is you can a bit narrow down your search to the one whose S11 or S22 close to 50-Ohm.
this may u may save time in designing matching network.