The MuseScore Orchestra SoundFont
Of the many SF2 SoundFonts in the world, one of the best for playing orchestral music is FluidR3/FluidR3Mono—which is why it's included with MuseScore. However, there are a few problems with it: the default piano is too "bright" ; a few notes in the violin are noticeably out of tune ; alto and tenor saxes are out of tune ; and the clarinet sound is awful .
MuseScore's ChurchOrganist Michael Cowgill has been gradually adding improvements in recent years to the SoundFont originally created by Frank Wen, having added an alternative "mellow" piano, currently putting in the hard work to manually loop 135 individual samples just to replace the clarinet sound, and most recently fixing an issue with the timpani.
I don't have the patience for that. ;-) So I fixed the above mentioned issues by borrowing samples from other SoundFonts. The result is, to my ear, already the most lifelike orchestra that exists in the SoundFont format—and I intend to improve it in the future.
It's frustratingly impossible to track the actual origin of the samples, but it's pretty near certain that you can use this for whatever you want.
So, here's my customized SoundFont, mostly FluidR3Mono but with modifications and replacements from GeneralUser GS and Timbres of Heaven. For me, this is the orchestra I want to hear when I use MuseScore, which is why I had to call it the MuseScore Orchestra SoundFont.
Below is an unedited MP3 of https://musescore.com/classicman/scores/887471 played with the MuseScore Orchestra (please compare and contrast at various points with the MuseScore.com default playback—the clarinet is highlighted at about the one minute mark):
So, download and enjoy, my friends! And let me know which instruments to replace next.