Rules of discussion:

1) We're looking for the definitive sound card, not necessarily the best. While the Roland cards sound awesome, I wouldn't say it's what first comes to mind (do not, however, let this opinion discourage you from making an argument for it if need be).

2) Be as general as possible. After all, different games sound may better with different cards.

Onboard squeeker, been around since the dawn of time and are still going strong.

Soundblaster 16.

By far the most common sound card I've seen in the 90s was the Sound Blaster 16. For games that didn't support it, telling the game that you had Ad Lib always worked in my experience, although I've never encountered an actual Ad Lib card.

Sound Blaster, no question. As stated above, almost everything supported it or AdLib.

Sound Blaster Pro. This was the card everyone cloned.

I love this setting, but I guess compatibility will be a (big) problem.
Many games only accept specific I/O address for specific sound device.
Plug and Play is not reliable in 1990s.

Your onboard sound card is probably more capable than any of the suggestions here. As I recall, Dosbox emulates a Sound Blaster 16 by default, but it can be set to work with others.

Sound chip emulation is pretty bad, no matter what software you are using.
Onboard sound chip today is VERY powerful, but the emulated sound/music for old games is nothing perfect.
That is why so many retrogamers record original music/sound with the original {PC speaker, sound chip, sound card, midi device}.

Probably because they pretty much took an AdLib card and put a bunch of extra shit on it. :P

I don't recall that being the problem.

Soundblaster16 used its default IRQ (5, or was it 7, I don't remember), base address (220h) etc. Not sure if I had to touch its MIDI port setting or did it even have one, but I didn't use it as a MIDI card so it wouldn't matter anyway.

Roland SCC-1 MIDI port address was 330h (the default). I don't recall what IRQ it used but at least it didn't conflict with others.

Roland CM-32L is an external MIDI module connected with a MIDI cable to the SCC-1 (which acts as a MPU-401 MIDI interface card for it) so it doesn't need its own settings at all. Depending whether I wanted to hear Roland SCC-1 (General MIDI/GS) or Roland CM-32L (MT-32 (LA-synthesis)) sounds from a game, I'd just mute the unneeded device with the external mixer. Otherwise they'd both be playing at the same time, translating the MIDI messages to sounds they thought would be the right ones. Of course if I wanted to use SCC-1, I could simply keep the CM-32L powered down (ie. not turn it on at all) so then I didn't need to touch the mixer either.

Gravis Ultrasound... yeah I recall I probably needed to change its default settings so that it wouldn't conflict with either SB16 nor the Roland SCC-1, but then as said there were only a handful of games where a Gravis Ultrasound was really needed, mainly some Epic Megagames games and some other pinball games. These used Amiga-style sample-based tracker music where GUS shined (if it was supported). I don't recall them giving a problem using GUS with its non-conflicting settings.

For the rest of the games which claimed to support Gravis Ultrasound (e.g. Doom and such), it was utilized as a mediocre General MIDI sound card + DAC. I already had Roland SCC-1 + Soundblaster 16 for that purpose so GUS was not needed for such games.

Wasn't the GUS popular in the demo scene though?

I have a SB16 and a GUS ACE in my pentium box. I recall there was a phantom gameport in the latter that needed to be disabled with software, or the gameport in the former wouldn't work.

Creative SoundBlaster Pro 16 + Yamaha XG midicard (attached to the main card)