I've had quite a few air rifles of my own since a mark 1 Meteor in 1964, and a few borrowed before that. One of the best grouping rifles I have ever had, and you can forget about EV2s and ProTargets and Walthers and FWBs etc, was a humble Daystate LR90.
This is a simple design knock-open PCP unregulated, a lighter weight version of the Huntsman. In fact I had two of them at the same time, and I had three barrels, a 177, a 22 and a 25 calibre.
The best combination was the early action and the 25 calibre barrel. It pumped out 26 grain Bisley Superfields at about 450 fps velocity and for any ten consecutive shots they would land on top of each other.
Today I have three rifles that are contenders. The Hammerli AR20 FT is my rifle of choice for bench rest, where you are looking to take out a white pin head at 20 yards. Daystate Airwolf for long range shooting, 60 yards plus, again bench rest but this time using 10.3 grain Exact Heavy pellets and the Westy single shot tray.
If I had to take a bet on group size for five consecutive shots at 50 yards, then the rifle I would select with no hesitation is my S400 with the export length cylinder and the barrel sleeve extension. Whatever the combination of vibrations and so on happens to be, I don't know. Exact Express pellets.