It's EWQLSC. That's Eastwest/Quantum Leap Symphonic Choirs, in case acronyms ain't your thing.
Competitors are few, but present. There is Symphony of Voices (the demos of which do not entirely impress me anymore... funny how they did years ago, but the library is quite old now) and VOTA (Voices of the Apocalypse, basically the precursor to EWQLSC, and still pretty decent, but with some licensing restrictions), and of course Bela D's wonderful children's choir Giovani which I have not used but everybody goes nuts over whenever it gets brought up.
The usual "it's all just opinion anyway," "your mileage may vary," etc etc, still applies but... EWQLSC is pretty much the undisputed leader of choirs.
"Voices" is something else however... I know jack crap about any world vocals libs, or any CD's with vocal licks on them or whatever...
But choirs, and EWQLSC in particular, I know much about. You get the expected SATB, and a boy's choir as well, and a couple of female soloists. The true magic comes in the form of the Wordbuilder. You know. Type in phrases, get the choirs to sing lyrics for ya. It's pretty freakin' cool.
If you don't have Sonar or Cubase, wiring all the required utilities together so you can get the choir to respond to both the notes you're playing on your controller and the lyrics you've typed into the Wordbuilder, can be a bit tricky, but I don't have either of those programs and I work with the choir just fine (I'm using it in Acid Pro 6 and energy-XT, the latter of which is an uber-pain to get working with the choir what with the open-ended modular design of the app, but it's certainly possible).
Ehh... not much more than that. Using the Wordbuilder as-is with no tweaking, can yield decent and even perfectly acceptable results if your choir's gonna be buried in a nice lush mix, but expect to do some serious tweaking in the Wordbuilder if you want absolute realism with the words being sung. Luckily, this tweaking mainly amounts to lengthening certain consonants' durations so they are more pronounced, boosting the volume of certain parts of your phrases here and there, and manually manipulating crossfades to get your words to flow correctly. It's pretty easy to do, and the software overall is very stable for me and quite fun to make music with.
You also get whispers, shouts, and some vocal FX (like what I guess is the vocal equivalent to glissandos and stuff). They can add nice dynamic variation to your choir in a piece and make clients go "wowzers, that didn't sound very sampled at all, that was super neato!"
Oh, and unlike its precursor VOTA, Symphonic Choirs can do light, quiet passages as well as massive cheeseball epic bombast. It's also been used to great effect by Mr. Roberto Ferrari to sing some baroque-style passages as well, so it's definitely flexible enough to be used outside of film scoring.
Go to the website, listen to the demos, weep openly for Nick Phoenix's awesomeness in producing this library, then buy it.
The URL being censored and such, I can't type it in directly. But here's a hint: sounds. online. dot. com.
Hope you find what you're looking for. I know I did.
EDIT: (obviously, the previous poster and I were posting at the exact same time) Here's my reviews and I explain which ones I own:
EWQLSC - East West Quantum Leap Symphonic Choirs - very expensive but very full featured and VERY big learning curve. However, if getting a massive choir to sing words you choose is necessary, this is almost the only package that can do it. I have yet to make it sound good. I have an AMD 64 3ghz processor and 2 GB of DDR ram and it is not nearly enough to run it as is (with the included Kompakt sample player). However, since I also have Kontakt 2, it runs very well in that. I have some weird issues with my midi master controller - but I believe that is unrelated to the actual software. You get men and women choirs as well as boys - all separated between sopranos, altos, tenors, bass and boys sections. Once I grasp the word builder utility properly, I am sure I will love this product.
VOTA - Voces of the Apocalypse - Older than EWQLSC and meant for a more dramatic (Wagner -esque) type sound. Gigastudio only (no included sampler) and has a 1st generation word builder utility as well (so it can sing what you type, but in the more dramatic "The Omen" type sound). I don't own this one so I cannot say anything good or bad - but the demos sound sweet (don't they all).
Bela D Media - Giovanni - Voices of the Young - Children's (Boys and Girls) Choirs - Must have either Kontakt 2 or Gigastudio. What can I say - simply the most beautiful, haunting and inspiring children's choir ever captured. I don't know how many other products have this, but none of them sound like Giovanni. I own this and have used this in productions. If you are looking for that eerie, haunting and beautiful sound that only a children's choir can produce for your next project - Giovanni is a masterpiece and totally affordable as such. Worth every solid penny. The Kontakt 2 scripting is also SUPERB!!!
Symphony of Voices - By Eric Persing of Spectrasonics - Sounds amazing considering it has been around since the early 90's. It is a staple of hollywood trailers and movies and you definitely have heard this collection before. It is samples only, akai, giga and some other formats I believe (but not Kontakt 2). It is pricey for it's age, but the quality and variety is through the roof. I don't own this one yet, but I plan on buying it. If you need background choirs, soloists, gregorian monks or enya-type sounds, this one works great.
Best Service #13 - Choirs - Don't waste your money. I bought this for $30 and I still feel cheated.
Peter Sydlacheik (I know I spelled that wrong) Choirs and Extended Choirs - dated, but supposedly very good. Mostly oohs, aahhs, eehhs, etc. Some soloists I believe, but for the money and if you don't need either VOTA or EWQLSC then get SOV instead (my opinion only). The demos never impressed me from this one, so I never bought it.
Vox'd - BITR (Back in Time Records) - a synth using the WusikSND format, but really not bad if synthetic choirs is more your style. Fun to play with and easy to program. Includes it's own VSTi synth and can be combined with Wusik, Manystation and BITR's TSW products. I have all of the above and use them daily. The best value bar none for such products and samples.
There is a collection of "Choral CDs" floating around on ebay and the like - I won a auction of this product for $10 including shipping from the UK to the USA (so I won it for like, $3 after conversion from pounds to US dollars) - mostly as a joke - The initial bid was $.99 so I bid $1.99 or something like that. No one else bid, so I won it. I still feel bad about doing it, but since the description was 100% vague, I had no idea what I was getting. Obviously beware - this cannot possibly be legal. I'll leave it at that.
BelaD's- Giovani Childrens Choir.
Spectrasonics- Symphony of Voices.
I have the Giovani, and am not too far off from purchasing the SOV. (Unless Eric and Frank have updates in store, then i'll wait. Please Eric and Frank, let's have a set of full blooded Germanic and Slavic male and female choirs. Something Wagnerian, at least. Please. Or Else. )
Listen to the demos, both soft and loud parts. Full bodied, gentle, and everything between.
Sublime sound, and wonderful tone, in just about every musical environment.