Most FT shooter use the Crosman Premiers or Beeman Kodiak Match pellets. The is no definite answer here. A lot has to do with personal preference and the velocity of the airgun you shoot. Heavy pellets offer more resistance to wind and that is their big advantage. However you take a big hit on the amount of trajectory you have to deal with.
Light pellets are best for lower velocity airguns with lower power scopes.
This will give you more error in range finding and still get a hit. If you have a higher power scope and can range find more accurately, you might go with heavy pellets. If you have a higher velocity airgun, use heavier pellets to lower wind deflections. A Standard Piston gun may have the internals of the gun tuned and modified, however the externals of the gun may not be changed, ie a new stock or external wieghts and the like. The focus adjustment on a scope is much like the adjustment an eye doctor uses when they test your eyes for glasses.