The Union Chapel Organ -- I forgot about that one! I bought it when it came out, which from the file system time date stamps looks to be fairly recent -- October 2018? That's probably why I haven't had a chance to put it to a full test yet, other than playing around with it on its own.
Looks like I placed it ever so slightly below Soundiron's Lakeside Pipe Organ (recorded nearby at lake Merritt in Oakland CA) and Garritan's large collection (which I rank high mostly for its academic depth and its breadth of coverage of different eras). VSL of course towers above the rest, due to its incredible recording quality and gigantic number of stops as well as ease of programming and mixing registers (the organ version of that terminology; not music staff version).
As a friend of mine who is a professional pipe organ player said many years ago: the organ is a family of instruments. VSL is NOT the only one I use in my collection, but I'm often pushed towards it when I need certain stops or combinations (registers). I even use Precisionsound's Knutby Church Organ sometimes, as it has a particularly warm timbre without being bright or too "big" sounding. Sonokinetic's Toccata is a bit different too (Bartholomeus Basilica, Netherlands). Too many others too list.
I should get back to the new one from Spitfire this weekend though, as I probably forgot to give it a final pass on some of my more challenging projects where I use 32 or more different stops.
Ive got a lot of organs - and I wouldn't say SF is my favorite organ, but it's an excellent one. Organs are really unique instruments - and each one is it's own living breathing instrument, so the differences between libraries really comes down to 1.) how well it captures it and 2.) if you like the character.
The room is just as much a part of the instrument as the stops/mechanics of it.