I've got the following methods of rendering non-redbook music:
OPL emulation
MT-32 (Old firmware)
Roland Virtual Sound Canvas (supports SC-88 Pro, SC-88, and SC-55 sound maps)

Rather than going by the subjective question of what sounds best, I want to know, what standard was the game composed for? If MIDI, then what device?

My understanding is that GM/SC-55 tends to be the de facto standard for games from 1993 and onwards, but there are some exceptions. For instance, I have heard System Shock is meant for OPL3, and it supports GM by essentially providing transcoded tracks. The fact that ScummVM doesn't yet fully support GM (or MT-32) for RTZ makes me wonder if it is also one of these exceptions.