That gun will probably shoot H&N Hunters most accurately, closely followed by H&N Crow Magnons, Barracudas and JSB of one or another weight.
You will get better penetration with Hornets, but you'll have to weigh that versus probably a bit less accuracy.
Anything that's "small game" enough for this gun should be a pass through shot with almost anything. Maybe avoid JSB Hades.
I have tested my Hatsan Hydra .25 pretty extensively with every pellet there is. My statement is assuming the Flash is almost identical but with just a little less FPS.
Also if you haven't bought a Flash yet, look at the Hydras. They have a bit more kick and I really love the lightweight wood stock and trigger. I'm sure the Flash is similar but generally 100 fps less power.
It is not my most accurate gun but it hits the spot for a 50 yard gun. I have a red dot on mine now, I rarely miss the target and it handles very well. I use H&N Hunters because the gun loves them.