Is this a field target or 10-m rifle?
If 10-m, then try H+N Finale match, RWS R-10, JSB Match, Size wise, you will have to establish that yourself by bench testing for grouping. Whatever size in whatever make gives you the tightest group, then thats the size/make for you.
I am testing H&N Finale match on my P700 alu. So far, 4.49 pellets seem to be doing better than 4.50. I am trying to test using the method described in MEC's "air rifle shooting" in which they use the scatt at 10 meters from the kneeling roll, instead of a fixture.