I found this cool youtube channel that features loads of MIDI synths, some old and newer ones. It is basically only piano sound comparison betwen these devices using a Chopin classical piece. In my opinion this is quite cool, check some of these known devices that could be used in some DOS games:

Casio CSM-1, used in some Sierra titles the piano sound is amazing for such an old device!

Yamaha FB-01, yep used in some more Sierra adventure titles as well and the basis for the IBM MFC, the OPP sound chip sounds great in the higher notes and not so great in the lower pitched ones. But hey this is FM synthesis afterall.

Roland CM-64 LA mode, this should be a pretty known community sound. This is a loved device but i think its piano sound is not the strongest point of the LA devices.

Roland CM-64 PCM mode, now using its CM-32P part which unfortunately is irrelevant for DOS games shows its strenght here.

Now for some GM devices:

Roland CM-500 GS mode, a pretty well known piano sound around here.

Yamaha MU-2000 XG native mode, wow sounds pretty awesome in XG mode. Too bad that no DOS games used XG devices natively.

Roland SC-8850 in 88Pro map, sounds pretty amazing as well.