If you want one, expensive unit, go for the Roland CM-500. It includes both LS and GS sounds. Be prepared to drop around $400 or so on it, shipped, and note that it does not have the screen for you to get the SYSEX messages from Sierra games (i.e. Insert Buckazoid)
If you want to get off a bit more cheaply, get an external MT-32 and an external SC-55MKII. You'll drop around $75 on the MT32, and around $100 on the SC-55MKII. You'll get LA sounds from the MT-32, and the extended GS sounds from the MKII.
If you want to deal only with internal cards, purchase the LAPC-I for MT-32 sounds, and the SCC1 for SC-55 sounds (or SCC1a for SC-55MKII sounds). If you've already got a sound card with a waveboard header, you can track down the SCB-15 or SCD-70 Roland wave cards and get SC-55/SC-55MKII sounds, respectively.
If you're playing early midi, such as from 1988 and prior, you may want to get an intelligent-mode Midi card. Examples include a true Roland MPU-401 card: an MPU-IPC-T with breakout box, MPU-401/AT (I would love one of these, but haven't found one yet), or if you're a microchannel PS/2 nut, an MPU-IMC-T card with breakout box. Non-Roland alternates include the Midiman MM-401 card and the Music Quest MQ32 MQX-32M. Note that guys on QuestStudios forums have found that these tend to work a little better in faster machines, and still offer Intelligent Mode (rather than UART/dumb mode offered by most sound cards)
Cheaper alternatives to the Intelligent Mode cards are just to use a plain old midi/joystick port on your favorite sound card, then use software patches to trick the games into thinking that you're using an intelligent-mode controller. IIRC, NewRisingSun has created several of these, and I believe that they're available over at SierraHelp.com
Also, some people have had great success in uploading MT-32 sounds to sound cards with external memory for uploading sounds. I haven't tried this, and don't even have instructions to do it (though I suspect that there's threads on QuestStudios about it)
If you're starting to get the idea that there isn't a perfect, one-synth-fits-all gambit for gamin, then you're right
If you're a gaming purist, hate hearing hanging/missing notes, then you'll probably end up investing in several synths... since games written to take advantage of bugs in the MT-32 gen 1 might not sound right when played with an MT-32 gen 2, or a MT-32 clone. etc, etc, etc....
I started out with an MT-32, and then added probably close to a dozen other synths covering the gambit from Roland though Turtle Beach and Yamaha, both internal and external, and really the only thing I'm still after is the Roland MPU-401/AT.... Welcome to the addiction