Acknowledging that every rifle is different, over an extended period of time I've used the following in my 452;
Lapua subs - by far the tightest groups with a subsonic round in my rifle.
Fiocchi subs - very good but groups are not quite as tight as Lapua (I am currently using Fiocchi).
Eley subs- very good results with most batches but quality can vary.
Winchester subs - very good, better than Eley (the older design Winchester and not the new heavier bullet).
RWS subs- very good accuracy but expansion doesn't seem so good.
As for absolute best accuracy from my rifle that was achieved with RWS high velocity and RWS R50, both non expanding rounds. Consistent 5 round groups using both at 100yds that could be covered by a 1" patch. That was some time ago before I had my 20" barrel chopped to 13" and I haven't used anything but subs since then. I don't think that my rifle is at all ammo sensitive. Good ammo and you get good results.