Once VSL has other articulations sampled (sordino, etc) I believe it will be the most flxible overall, or in line with the upcoming QLSO libraries strings. Atleast in terms of sound. Possibly performance.
VSL, being recorded in its silent stage, allows for you to do alot of mixing/coloration if you want with out trying to "remove" a certain character. It also takes very well to reverbs.
SI is very colored, but its a very nice sound, even without any legato type tools it can sound very realistic.
GOS's legato tool is outshined 10 times by VSL's performance tools. However with the slides and other samples you can add some extreme realism. It is however, also colored in its own way.
The upcoming QLSO library will have many options for flexible sound. Multiple mic positions, allow you to use the natural ambience to color the sound, or you can use reverb+EQ on the closer mics to create more of your own sound. Its still yet to be heard whether or not the legato features will be up to VSL standards tho. I doubt it will be bad tho, in fact I expect it to be quite good.
In my opinion your best bet is to go with one of the libraries, then use some solo string samples layered on top to color and create and ensemble sound you like.
Dan Dean solo strings layers well with libraries, the upocoming Kirk Hunter library and London Solo Strings libraries should also work nicely.
There is no "one string library to rule them all", your best bet if you must choose is based on your needs.
Since you seem to want flexibility, I suggest VSL with the performance package, and some solo strings, or one of the "lite" versions of SI or GOS. The performance package in VSL is possibly the first thing I'd buy if you are going to mix and match and if you need to save money. Using the legato and repetition samples will get you pretty far layered with other libs.
once the solo strings from VSL is done, they will be another option for coloring. Solo samples tend to let you add a more expressive vibrato over the top of ensemble vibrato. Like a first chair player emphasizing a part.
As of right now there is no solo library out that has a very expressive piano sound. KH is selling early/beta versions of his library, and I'd expect that one to have some. Its quite cheap too, so it may be worth it to you to invest in that one.
The nuances added with this technique is uncanny, its just that it eats up some more poly. As much as I like the VSL strings, I cant wait to have the solo strings to layer on the top, or smaller ensemble variations to layer on top to add more vibrato and different character.
Again as I said, I think VSL in its final form will be the most flexible, its just about if you want to wait and update.
That all said, I sitll like my new updates for GOS alot [:)], totally unusable for some music, and lots of problems if you pick at them, but they have a sound you cant get with any other lib if you ask me.