Lately I tested as many soundfonts as I could find. Both on my X-Fi card and with BASSMIDI software driver.
So far the best General MIDI soundfont I found for gaming is Giant Soundfont which can be found at http://giantsoundfont.npage.de
The name suits it, because it is rather big, about 540 MB uncompressed, but it sounds really good and what is more important is still actively developed and patched by its creator, and developed in a way suitable for gaming.
I have made a few sample videos with it on some old games via dosbox
AdLib Synthesizer vs. Giant Soundfont https://www.youtube.com/watch?v=He_mlHj7tOU
Lost Vikings intro/tutorial https://www.youtube.com/watch?v=FvWPXly_yVw
Blackthorne intro sequence https://www.youtube.com/watch?v=3IG3MwjPSj4
This is another huge soundfont Evanessence, which is close to 2GB. https://www.youtube.com/watch?v=vJ86HUs61k4
Giant soundfont sounds better than evanessence on most of the games, except for some reason on Westwood games, where Evanessence is better. I think currently Evanessence is reduced and is called Musying Kyte or something like that. Havent tested the quality after the change
Dune 2 intro, using Evanessence soundfont https://www.youtube.com/watch?v=vJ86HUs61k4
Interesting. I have Musyng Kite, Giant 6.0 beta and Timbres of Heaven 3.2. Tried comparing all 3 in Dragonsphere and Flight of the Amazon Queen and while I cound hear the difference, couldn't decided which one was better. Eventually settled on Musying Kite.
As was mentioned by someone else, there are also soundfonts which aim to reproduce the SC-55 sound although they're questionable legality as they often use Roland samples. (But it isn't always totally clear whether the samples used on free soundfonts which don't aim to sound like the SoundCanvas were obtained from an acceptable sources.) Besides Patch93, StrikingUAC/Trevor James is popular. I tested both, but decided I somewhat prefer the other soundfonts although again it was hard to decided. There is also a 1.8GB commercial soundfont but it costs like US$90 so I think few people have tried it. I imagine one limitation is even if you use only SoundCanvas original samples, the synth probably isn't going to processes them the same as the SC-55 did. The Sound Canvas app in iOS is said to be quite a faithful reproduction, but not something you can easily use for gaming.
Of course to a big extent it depends on your personal preference. Some people may prefer the music to sound as close to what the original composer may have been aiming for. Others may care less about this and just want something which sounds good in their opinion. As has been mentioned by others, balance between the instruments or rather a similar balance (not necessarily the same since to some extent that's also personal preference) to what the composer was using is ideal but beyond that it's up you you what you think the instruments should sound like IMO.
One factor which is often not talked about is that even if a composer was primarily working with an SC55 or whatever, I imagine some or may be even many would have at least tested, and probably tweaked their games to make sure they worked ok with other popular GM synths, cards and daughterboards common at the time including non MPU401 ones. E.g. Gravis Ultrasound and Soundblaster AWE32 (depending on when the game was released and what was popular at the time).
Edit: A bit more testing and checking back found several other Soundfonts I'd tried or seen recommended before. There is WeedsGM3, ChoriumRevA abd SFM v2.01. All these are fairly old but some people find them good for games and old doesn't have to mean bad. There's also Arachno Soundfont 1.0 which is somewhat inbetween, last updated 2012 I believe.
Personally, with my limited testing I couldn't say any of these were definitely better than what I was using.
There's also Crisis General Midi. 3.01 is fairly old now 2006. The author was developing 4 but abandoned it due to the lack of donations or licences. However the author later said they lost their job and so has more time to work on Crisis 4. I'm sure we all hope they will get or got a new job or whatever, whether or not they have time to work on CGM4. (Someone also suggested they consider crowdfunding. I think this person mentioned this elsewhere on GOG.) Something to keep an eye on in any case perhaps.
I probably should have mentioned, of the earlier soundfonts not counting the Soundcanvas ones which seem to be developed by different people for short times, both Giant and Timbre of Heaven seem to be under some degree of development. For Musygn Kite the author has said is the final version of the soundfont (which has gone under many names), but welcomes others to develop it. It sounds like it isn't the first time the author lost interest so it's possible things could change there too.
You can find a fair few comments and criticism of the various soundfonts when it comes to games in various places like VOGONS and here, where people feel the strengths and weakness are. For the older soundfonts without many or any versions these criticism are probably still valid although you may not agree with them. For the ones like Giant or Timbre of Heaven, it's possible things have changed. (I guess one advantage of the changing names for of Muysng Kite is it's clear what version people are talking about.) Where people have specified where exactly they found problems you can probably test this yourself. (If it's a specific game, you can probably find the midi files somewhere if you don't own it.)
Finally perhaps useful to remember you can combine soundfonts. At the simplest level, with Virtualmidisynth or any other Sountfont 2 synth which supports loading multiple sountfonts, you can load supporting soundfonts to provide instruments you ons is weak in. E.g. Drums by Slavo seems a popular addition to some of the older sountfonts. Bellatrix Orchestra is another possibility. You will need to check how your synth handles priorities. And there's also the problem of balance again so you may have to edit one of your Soundfonts. (Probably the same if you only want some of the instruments although IIRC the X-Fi GM or something like that allowed you to select which instruments you wanted from the Soundfont.)