If you want to only listen to songs and you want to stick with Roland (as they're pretty much the de facto) and want real hardware, the way I see it is you ideally want between 2 to 4 modules, maybe 5 if you're picky or concerned about playback issues.
MT-32 (old, without headphone jack): This module is ideal for games that exploit errors with the MT-32 which will not sound correct on the newer unit.
CM-32L: This is the newer (with headphone jack) MT-32 with extra sound effects that some games utilize, you just miss the front panel screen and buttons. Ideally you want ROM 1.02 to avoid a clicking bug with earlier ROM versions. CM-64 is another mention but you'd need to mute the CM-32P part before playback to reduce noise and extra notes playing that shouldn't be, though useful if you want to listen to some Japanese game music that utilizes both parts. CM-64 is 100% compatible with CM-32L.
SC-55: This one gets tricky depending on what you want, as there are several iterations with noticeable differences. The most "compatible" that people have decided on is one with ROM 1.20 or 1.21. Other versions may produce issues with some build engine games, though these problems seem to arise from the composers not programming their MIDIs correctly or the game itself not sending the correct messages to the unit. The units using ROM 1.00 and 1.10 do have one patch mapped differently on instrument 122, being "Fl.Key Click" instead of "Breath Noise", so maybe try to avoid those units if you're concerned. For the most part though, whichever one you get should be fine, regardless of ROM version, as the MIDIs that break are extremely few.
SC-55mkII: An upgrade of the SC-55, featuring more sounds and higher polyphony (more notes can play at once), and a slightly higher quality DAC. This one is basically interchangeable with the SC-55 and comes down to preference again. My personal preference is this unit over the original.
SC-88 / SC-88 Pro: These units are significant upgrades over the SC-55 / mkII, featuring more and higher quality patches and even greater polyphony, though a lot of people prefer the sound from the SC-55 over these units. The SC-88Pro is a direct upgrade over the SC-88 and is 100% compatible with it in SC-88 mode because of the same hardware. A lot of Japanese MIDIs were designed specifically for the SC-88Pro. These units are not 100% compatible with SC-55 modes but are fairly close, due to differences in internal hardware.
Other mentions: The SC-8820 has maps for the SC-55, SC-88, SC-88Pro, and it's own map, but it is not 100% compatible with any of them (except for it's own map, obviously). This is because the hardware is significantly different and the DAC is even higher quality. The earlier Sound Canvas units are known for their somewhat muddy sound stage, and composers (mostly Japanese ones) that tried to compensate for that will have their MIDIs sound too off on this unit. The SC-8850 is best avoided as it can't play back any MIDIs accurately at all unless they were designed specifically for it. The SC-D70 is a digital output version of the SC-8820 and features the same incompatibilities, except for an even higher quality DAC. The CM-500 is not recommended either due to the vibrato being noticeably faster (sounds really weird).
tl;dr: Considering everything I mentioned above, my idea of the "cover the most ground" with Roland units would be:
CM-32L / CM-64
SC-55 and / or SC-55mkII
SC-88 / SC-88Pro
For a cheaper and more simplified setup (but not covering as much ground), I would suggest:
MT-32 (old or new)
SC-55 / SC-55mkII
Of course, there are cheaper versions of these units as well that basically remove the front panel screen and buttons but still sound exactly the same as their respective modules. If you don't care about any of the front panel stuff, you can save some money here. There are also rackmount versions of these units if that's your thing.
My setup, which I'm very happy with, uses an MT-32 (old), SC-55mkII, and SC-88Pro, though I would like to add a CM-32L / CM-64 but those units are incredibly expensive for the few that are listed as auctions. I do also have an SC-8820 (my first unit), but I don't really use it anymore.
Pentium MMX 233 | 64MB | FIC PA-2013 | Matrox Mystique 220 | SB Pro 2 | Music Quest MPU Clone | Windows 95B
MT-32 | SC-55mkII, 88Pro, 8820 | SB16 CT2230
3DFX Voodoo 1&2 | S3 ViRGE GX2 | PowerVR PCX1&2 | Rendition Vérité V1000 | ATI 3D Rage Pro