Since Piccinesco has entered the discussion quite forcefully (just kidding...), I feel like giving my two cents on the various libraries, sticking to strings for the moment.
I have had the opportunity to try and use the following libraries: Garritan Personal Orchestra 4 (GPO4), East West Quantum Leap Symphonic Orchestra (EW), on the Kontakt engine, not the subsequent version on the Play engine, Cinestrings, 8Dio Adagio Strings, Vienna Chamber Strings (solo strings), Vienna Horizon Opus 1, LASS 2.0, Berlin Strings, Embertone Friedlander Violin and Blakus Cello (among other things, I am in contact with the person who developed it), Symphobia 1, 2 and 3 (Lumina), Cinematic Strings (not to be confused with Cinestrings).
Unfortunately, I am missing the East West Hollywood series, which I haven't had the chance to try and therefore cannot comment on.
Given that expressing an opinion on all these libraries would be very long and perhaps boring, I will try to be concise by highlighting pros and cons based on my personal taste and some objective elements, so take what I say with a grain of salt.
Currently, among all these, my choice falls on LASS 2.0 regarding sections, while for solo strings the Vienna Chamber strings are certainly remarkable even if they are a bit dated, but 8Dio is also a good choice for solo strings.
LASS has the advantage of having a very fluid legato, which works well even for relatively fast passages; furthermore, the dynamic control with the mod wheel is very good and, if desired, can be edited as you wish. There is the possibility to create divisi in a realistic way by loading three different samples for each single articulation (e.g., leg A (4 players), leg B (4 players), leg C (8 players) total: section of 16 first violins) and if a three-note chord is played, the software automatically distributes one of the three notes to each sample; the final result is very good. The three samples are slightly different in terms of intonation, color, and response to controllers, which greatly benefits realism. According to some, the LASS sound is a bit "rough," "coarse," not soft and round like the EW, but for me it is more realistic. LASS is dry, so you need to apply good reverb to it. The samples are already positioned stereophonically, but that doesn't mean these settings cannot be modified. There are also FULL MIX samples, which are already a mix of the three individual samples, but, since space is available, it is preferable to use the three separate samples for each articulation.
LASS solo strings are not quite as good.
Lately, Berlin Strings has become very fashionable, the latest born as a string library. It's excellent, but I'm not 100% convinced. It has a unique "spiccato" sample that is truly remarkable, but the legato is a bit artificial, with a sound tail cut that is quite unnatural in the transition from one note to another. Furthermore, the legato sample alone weighs 4 GB, which makes it usable only with very powerful software. It has many articulations, perhaps too many; some seem like repetitions or unusable to me. Berlin Strings is recorded in the Teldex hall in Berlin, so it is already quite wet.
8Dio Adagio is based on a different concept: they focused heavily on various types of legato to make the library "emotional" rather than realistic. In the version I have, some samples have evident sampling errors, sonic artifacts, and strange noises that make the samples unusable (surely in subsequent updates they will have resolved this problem which was very obvious); moreover, the bad thing is that it's the user who has to adapt to what the library does best; if you try to do something else, the result is not good. If you listen to the demos, they will probably sound like the best ones out there, but actually playing a phrase with those samples is not easy. LASS might be less "emotionally engaging," but you can play whatever phrase you want with it; then it's up to you to make it musical.
Strangely, in 8Dio, solo strings are better than sections; they are much more playable and the timbre is not bad at all.
In my judgment, and not mine alone, the top in terms of realism is achieved with Vienna. Given that the Vienna libraries I have are a bit dated, but they have a very particular charm, a "seriousness" that allows you to feel safe; you don't have to worry that a certain phrase with that library might not work—if you use Vienna and a phrase doesn't work, it's because the phrase isn't good, because there will certainly be an articulation that allows you to bring out the phrase you have in mind. Vienna is a guarantee in terms of realism, even if lately the trend is that one can do without realism in virtue of the "epicness" of the piece. Lately, demos must have 24 (!) horns playing in unison, choirs doing things that no choir could do in reality, etc... well, Vienna shuns all this; Vienna is
for the sake of realism, and I like this thing very much, because it is useless to develop credible libraries if then one realizes things that are "incredible" (in the negative sense of the term).
For the moment I will stop here so as not to weigh down the post too much; I promise myself to complete it in a subsequent post talking about Cinematic strings (quite good), Cinestrings (doesn't convince me much), Embertone (I advise against it for various reasons), EW (honest but now outdated), Garritan (a toy compared to the others, good for Finale and nothing else)......