If you are a professional movie composer you likely have quite an investment in computers and detailed sample libraries. These libraries are often priced in ranges that make them prohibitive to the beginning composer or even a more seasoned composer who isn't presently funded in any way for their work.
Though it should be said that most of the better sample library companies offer entry point software, sometimes for free in order to get you started. These beginner packages often don't offer enough capability for serious composition. This is the beginning of a long expensive road to acquisition of everything needed for large scale orchestra composition using these libraries.Some companies have started a monthly rental fee to make using these libraries more attractive to more composers.
Thankfully there are products that fill the gap between those libraries and soundfont/rompler kinds of sounds that often just sound cheesy and unrealistic when coupled to a composition you have worked hard to make.Some of your choices might be limited to either use of a DAW for composition or notation software. DAWs offer more flexibility here over the end result of the sound in particular. Many notation programs offer sometimes passable included soundsets or the capability to mate the notation program to a DAW which is in my option, the best of both worlds if you write notation. Much of this depends on the composers personal standards with respect to the quality they demand for their work.
Thankfully there are sound libraries priced at points much more attractive to the budding composer that faithfully reproduce a good facsimile of the instruments. Since this is mostly a site dedicated to orchestral compositions both small and large I will omit the smaller instrument dedicated libraries and save that discussion for another time. Instead I will concentrate on the "all-in-one" kinds of libraries that a composer will need in order to make entire orchestral mock ups. It should be noted that it is of prime importance to make sure your computer specs either meet or exceed the expectations of the software requirements. Often the minimum spec isn't enough for serious work and so I recommend a computer that far exceeds these expectations. This is also a subject for another time. This isn't to say you need the very best, but old OS using 32 bit programming languages on computers showing some age are probably not good hosts for this sort of thing.
In order to use these libraries one either needs notation software that hosts them or a DAW. Many Libraries also require a program made by Native Instruments called Kontakt since it is the shell they load into. Here's a plug of one of my favorite free DAWs- Cakewalk by Bandlab. This program is miles ahead of anything else free out there.
Here are the libraries not listed in any particular order.
Miroslav Philharmonic- Note Kontakt is not required.
As a general rule these libraries sound ok. Some better than others What they often lack in comparison to their more expensive siblings are mic positions and less command over the sounds. Sample quality sometimes isn't as good.
Having said that they are excellent in many composition settings and work well as sketch pads or examples that sound mostly realistic in comparison to sound fonts and often are useful in some professional applications.