I was quite surprised to not see this as a suggestion yet after searching through, so apologies if this was already mentioned elsewhere!
Star Control II is a constant not only on my list of “Best DOS Games”, but on my personal “Best Games Ever”. A completely different beast than its predecessor, it has an amazing combination of exploration, adventure, and good old fashion spaceship blasting. It also manages to be a rare breed of game that can simultaneously be incredibly funny and deeply serious.
I play through it again every few years myself just for fun, though it’d be a joy to see others giving it a whirl. And in terms of discussion topics, it’s got a whopper of a history for the franchise following it, culminating in the recent announcement of the newest entry in the series by the original creators. Certainly plenty to talk about!