That’s a very vague question. In theory, good ammunition is uniform and consistent. Meaning the bullets are identical in weight and size. The cases are all the same length (even weigh the same), they all have identical powder charges, and overall length is the same. This type of ammo is called “Match” ammunition and its purpose is to put every round through the same hole. Manufacturing match ammo is a science in itself. Consistent is the key word. Shooting match ammunition is somewhat senseless unless you shot it through a good quality match barrel.
Another angle could be in what it is that you’re attempting to accomplish with it. Are you going to shoot holes in paper, flesh, through steel? In this scenario you’ll have to contemplate terminal ballistics. What the bullet does upon and after impact. You have to select the proper ammo for your intended purpose.
Now my idea of good ammunition for practicing consists of it being cheap (top & foremost), clean, reliable and relatively accurate.
For self defense, price is not an issue (realistically speaking), but reliability, accuracy and terminal ballistics are (huge).
For your new carbine, you’ll have essentially two options; target ammo and self-defense (premium) ammo. You’ll notice the difference in price tags, the later being considerably more expensive. As a rule of thumb, full metal jacket (FMJ) is not good for self-defense; you’ll want ballistic tips like Hornady TAP or Federal TRU.
Probably more than you asked for, but I was bored. I’m sure you’ll get a lot more ideas from others. Welcome to the sickness