That can't be answered without a specific purpose in mind. Generally speaking, brass cased rounds are more desirable than aluminum or steel cased rounds. For plain target ammo, personally, I used Winchester white box and CCI Blazer brass.
If they haven't given in to the price gouging craze, the aluminum cased CCI ammo used to sell pretty cheaply at Wal Mart. Other than that, gun shows and gun shops are your best bet.
The glock 21 fires the .45 automatic colt pistol (.45acp, or .45 auto) round.
There are varying opinions on what ammo is the "best." Generally speaking, the best ammo to use is whichever ammo is the most reliable in your particular gun.
Federal premium ammo
Both can refer to 45ACP. 45 alone can also refer to 45 Colt
Not sure where you got your information from. Define inaccurate. I've shot 45ACP 230 grain ammo out of a WWII M1911A1 that would put them all in the chest of an E series target at 25 yards. All weapons will exhibit a preference for one or more types of ammo/loads. You have to experiment.
The ammuntion it is chamberd for.
Idaho potatoes
I have had good success with Federal rifle ammo.
No.
A Colt Lightweight Officer's Model in .45ACP caliber. It is small and light and carries seven rounds.
Gold Dot