Forster's primary sizing dies (those with an expander ball) are very good, and I use several.
But aside from being well-made, the other reason they result in straighter case necks (and hence loaded rounds) is the expander ball's high up inside the die, just below the neck portion of the die. Thus the neck of the case is still inside the neck portion of the die when it's pulled back over the expander, so the case is forced to remain in-line with the die. This definitely results in straighter necks than standard sizing dies where the expander ball is down near the bottom of the case, where necks can pull out of alignment.
That said, if the case necks aren't very even in thickness the neck alignment can still be a little off even with Forster dies, but generally not more than .002".
Through experimentation I've found other makes of full-length dies produce very straight cases IF the expander ball's raised to just under the neck portion of the die. I've found basic RCBS dies can be adjusted to do exactly this--though of course decapping the case requires another step (one reason a Lee decapping die permanently resides in one hole of my turret presses).
I tend to use bushing dies when loading bigger batches of brass, or Lee collet dies. But there's no doubt that "hand dies" produce the very straightest brass, whether the basic Lee Loaders, or more sophisticated dies like Wilsons. Have loaded consistently sub-half-inch (5-shot groups) .22 Hornet ammo with Lee Loader "hand" dies.
The best bullet alignment results in any of my "press dies" comes from Redding Competition bushing dies in 6mm PPC, with sorted and neck-turned Lapua and Norma brass. Bullet runout is a maximum of .0005", one reason 5-shot groups from my Erhardt benchrest rifle average well under .2 inch at 100 yards, despite my less-than-perfect bench techniques.
