There are quite a few softsynths around, aren't there? Here's a list of ones i've tried, and good/bad about them. nearly any of these can make great midi if they are the synth the midi is composed for, but my focus is gm/gs/xg.

1) libadlmidi. It sounds JUST like windows 95 on a sb16. You can get better polyphony by adding more chips. actually supports portamento, too, as well as modulation wheel, pitch wheel, and quite a few other effects. even brightness. a number of old win95 games have midi composed for this, so it's worth getting.
2) libopnmidi. makes your midi sound like a genesis. 😀 all the same features of liboplmidi. sadly there's no libopmmidi yet to make your computer sound like a cps1 yet.
3) gxscc. sounds like a Konami SCC. i think it supports chorus and reverb maybe.
4) emu de midi. crazy mismash of retro computer sounds. not too impressive
5) fluidsynth. loads sf2s, supports everything in gm, plus portamento (which the creative cards did not), reverb, and chorus. can load "gs" and "xg" soundfonts, but doesn't understand everything a real gs or xg synth will. quality dependent on your soundfont.
6) ms gs synth. only supports gm, pitch wheel, portamento, and mod wheel. no chorus or reverb, gm instruments and drumkits only, even though the sc55 mk2 variation instruments are there. commonly converted to sf2 for use with other softsynths. a fair number of midi soundtracks work well on this, but we can do better now.
7) bassmidi. also does sf2. pretty similar to fluidsynth. can load a VSTI as well.
😎 virtual sound canvas. better than ms gs wavetable but not by much. mulitple version exist, and some of them are available as vsti.
9) hypercanvas. this is gm2, not GS. will not play GS midis properly, but will do GM2. vsti exists.
10) Sound Canvas VA. this is the one that doesn't suck. VSTi only. emulates multiple sound canvas modules, and actually gets right in the night GS version (with filter sweeps) correct. will play back most GS electronic dance music right or close enough. if you don't own it now, you must pirate it. Even if you do own it, you may need to pirate it anyway depending on your OS.
11) s-y20xg. silly gm only softsynth, but will play back most gm composed for sound canvas well.
12) sgp2.dll xglite softsynth with higher quality samples. doesn't support full XG. used in midradioplayer. better idea than s-y20xg for sure.
13) s-y50xg. available as a vxd (win9x) a wdm (for nt/2000/xp) and a vsti (for everything else). extremely difficult to buy now (vsti comes with SOL2), but wdm version can be downloaded from MS and installed for free. all versions except vsti have high latency. plays back midi composed for mu50/80 great, and most midis composed for sound canvas well. supports sound canvass nrpm in gs mode, and fully supports xg. came with many soundcards. Very difficult to buy now, and very easy to pirate if you need the VST.
14) s-y70xg. 20+50=70. Not kidding. you can switch between s-y20xg and s-y50xg. comes free with final fantasy 7 pc disc release, vxd only.
15) creative wavesynth. 9x only, came with creative soundcards only. has some interesting samples, including a synth brass that sounds like it came straight out of "Tonight is the Night" by Le Click, but otherwise not very good.
16) ymf-754 HiEnd. this is same as s-y50xg, but the samples are upscaled. Sounds the same, but may be more stable than s-y50xg on your system. pirate only special. hard to find now, but not impossible.
17) TyRUS. Much bigger and high quality samples. presumably ripped from a tyros. Sounds nice, but screws up stuff composed for mu50 and sound canvas at times. pirate only special. good luck finding this now.
18) Vampire. bigger samplebank than tyrus. screws up more stuff composed for mu50 and sound canvas. at least this one is downloadable if you know where to look now. used to be donation only, but someone finally leaked it.
19) wingroove. does a lot with a small sample bank. does a decent job on some sound canvas composed stuff, but we can do better now. still downloadable, and someone made a soundfont of it.
20) nuked-sc55. this IS an SC-55 in software, but is standalone only. usable with a virtual midi cable, and there is a CLAP plugin that can load it now. the sc55mkii pcm chip still needs to be decapped, because it fixes a bug in the filters that the mk1s have.
21) munt. it's an mt-32. nuff said. useless for GM, but many classic games were composed for this. a vst version exists that can run two copies to "handle" gm.

The following ones i haven't tried out yet.

1) yamaha tyros 4 VST. Expensive. What tyRUS wishes it was. Haven't been able to try it, because i'm poor. there's also a kontakt instrument of the tyros 2.
2) Montage M vst. only way to get this is to buy a montage M. Or pirate it using very sketchy cracks. Extremely large, needs beefy hardware, but can probably do a decent job playing most midi files. super expensive. as i'm not made of money, i haven't actually tried it.
3) coyoteWT. this is a simple vsti that loads gm.dls. This gets you the missing GS variation instruments, and almost certainly chorus and reverb as well. came with older band in a box, impossible to get now. haven't been able to try this. i believe it was a DXi.

Anyone else feel free to add their experiences and suggestions with gm/xg/gs softsynths.