You have to define what you mean by "best". All weapons will exhibit a preferecne for one or more loads based on bullet weight, profile, charge, etc..
Federal premium ammo
Caliber is .45 GAP (Glock auto pistol)
Check your owner's manual or call Glock.
.32 Calibre Ammunition?
yes
Yes
What does your owner's manual say? Don't have one? Call Glock and they will send you one.
Yes.
The glock 21 fires the .45 automatic colt pistol (.45acp, or .45 auto) round.
.45 ACP
.40 S&W with JACKETED bullets. The polygonal rifling in a Glock does not mix well with lead bullets.
No - they are very different cartridges.