Sound Blaster vs Adlib shouldn't matter as they are both OPL2, but Sound Blaster Pro as an option adds stereo sound and Sound Blaster 16 as an option adds 16-bit digital audio. Also DOSBox's OPL3 Emulation is good but not perfect, I would look into patching it with NukedOPL.

As for MIDI, you don't need a MIDI module for the best MIDI music in Dosbox. To the contrary, I think sound canvases are fairly overrated, there are many better MIDI modules available (XG modules, for example). Regardless, in a fairly modern PC just installing VirtualMIDISynth and using a balanced high quality soundfont like Musyng Kite (which is 1GB+ in size compared to the IIRC 4MB sample set of the SC-55) will yield vastly superior results for no investment.

appiah4 is pretty much on the money. You can also emulate a Gravis Ultrasound in Dosbox which is good for many games and scene demos. As for what is best, it varies from game to game, depending on what devices they were composed for. Experiment and see what you like.