What do you think the best (most accurate) pellet is for a 124?

The AA8.4 and JSB 8.4 are the best in my 124. Although there are not many that are bad in the 124.
Tried to add a pic. but don't think it worked. It is a pic @25 yds 10 shots with AA 8.4 of less than 1/2".

jsb 8.44 in mine

All guns are different. 2 guns of the same brand may like different pellets. It depends on the run and the number on the run as to the bore size. As dies and tooling wears you get different production. And Qc standards has a go and no go. Also the pellets run the same way. Many tins have many odd ball sized pellets. 
I got guns that shoot the crossman's tins like magic and maybe the same brand gun and caliber that will not shoot them. 

Then you got pellets that are made in the same factor under another name. Some times these pellets are made with XYZ dies but shoot better or worse than the factory top brand. So claiming the best pellet is a broad speculation. It's a personal preference dictated by the gun not the gunner. JMO

 RWS Superdomes shoot very well out of mine.

My FWB124 seems to shoot all quality pellets quite well, however gun seems to really like the JSB Exact !0.34 grain pellets, gun just loves them but also likes 7.87's and 8.44 Exacts. I installed a JM kit and the heavier pellet really tames the shot.