Garritan Personal Orchestra is great because it's a full orchestra on a budget. The sounds are good, not amazing. The sounds get looped after about 1 second on most instruments and there is no control over the vibrato on any of the strings. There are 3 solo Cellos which are all decent but the lack of control over is very anoying. You either like the sound or you don't.
Synful is a physical modeling VSTi that takes a sh*tload of training or programming to get a realistic sound (and that is a very big understatement). If you have the skills, I think it can be really good, but I don't wanna have to spend that much time.
On the higher end you have East West Quantum Leap libraries, Vienna Symphonic libraries and Garritan Orchestral Strings. They are top-notch, but be prepared for lots of work if you really wanna get it right. There's also Miroslav, the new MOTU philharmonic and tons of others, but I can't comment on them much.