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.