I have a "bunch" of Anschutz silhouette rifles. I did a pretty big test with 6 of them last spring.
Like most of the posts say, it depends on the rifle. Overall, CenterX shot the best groups on average, with SK+ coming in second place. In some rifles, SK+ shot better.
The "best" rifles shot TAC and CCI std well, but of course, SK+ and CentrerX shot better. Lesser barrels showed more specific liking for particular ammo. Some shot TAC better, some shot CCI better. TAC always had fliers. I also ran ancient Eley black box and RWS rifle. Since that ammo is limited in my stock, I just shot that for fun, but it followed the same path as the other ammo. Good barrel shot everything well, lesser barrel was finicky. My best barrel is a Douglas on a 54MSR sporter.
I shot multiple 10 shot groups at 50 yards to weed out "poor" ammo. Then I took the better results to 100 yards for multiple 10 shot groups. The results at 100 yards were pretty obvious, and a lot more telling than 50 yards.
For me, shooting offhand, SK+ is the best choice. It ran a definite good race with CenterX, and even bested it in some rifles. This might hurt some people, but personally I am better off practicing and match shooting with SK+, than dedicating my time and money for .1 inch better groups at 100.
I guess it is all relative, because some of my rifles # 1 ammo does not shoot as well as my best rifle's #2 ammo choice. So like everyone said, the rifle picks the ammo, and lot to lot the results may change.