Most Realistic Soundfonts?

See the section on Soundfonts in the handbook. It lists a couple of alternatives, including FluidR3, which is what I normally use. It is more convincing for some instruments than for others, but overall it is the best of the General MIDI soundfonts I have found. Unfortunately, while General MIDI is a convenient standard, it is not great for orchestra music - it lacks several important instruments (eg, bass clarinet), and doesn't define enough of the variations one would want to hear (muted horn, various bowing styles). A Google search will turn up plenty of non-General-MIDI soundfonts that are supposedly optimized for orchestral use, but since they are not General MIDI compatible, that means you'd have to customize your instruments.xml file to use the proper patch numbers, and I haven't found it important enough to deal with. So I use FluidR3, have bass clarinets play as bassoons or some other instrument, and don't expect to hear playback of an effects other than pizz and tremolo for strings and a muted trumpet.