Depends if you hunt with a dog or not, and if the dog is a pointer or flusher. I hunt with a pointing dog and my shots are generally all within 30 yds. No need for shot bigger than #6 or more than an ounce of shot. Most of my hunting is with a 16 ga. and IC/Mod but have also used a 20 or 12 and have done as well.
Friends who hunt without dogs or with labs and spaniels use tighter chokes, usually modified, or mod/full and heavier loads. Some use as large as 1-3/8 oz of 4's which makes sense if your reaction time is a tad slow or for wild flushing birds that must be shot as far as 45 yds. Beyond that is too far for any shot/load combination IMHO, far too many try to shoot departing pheasants at long range and see a few feathers fly, a leg drop, and the bird pumping on over the horizon to die slowly and be wasted. A going away and rising pheasant has its vitals covered by the hip bones/sacrum. That's why shot smaller than 6 does not work consistently well, not enough penetration through bone.
As mentioned before, nickel plated shot is a benefit if you can find it. Nickel 6's are deadly. Penetrate as well as regular lead 5's and better pattern density. It is fashionable to use very fast loads lately. I believe that about 1250 fps is optimum for good hard lead shot to pattern evenly. Faster loads do not penetrate appreciably better, kick more, cost more, are louder, and pattern less evenly. Most shots at pheasants are angling away, so apparent forward allowance is insignificant difference between the super fast and more moderate speed game loads.
Good luck hunting pheasants!