Thing is, it's sadly very variable. For every Titanfall 2 'Trial by Fire' masterpiece there's 100 other games with poor mixing in general or that barely use the surrounds.
If there's something you want to demo for 5.1 I'd say start with Titanfall 2. Anthem & Shadow of the Tomb Raider have great mixing as well, Eidos in particular seems to pay it more attention than normal. Doom 2016 is good as well. The Witcher 3, Kingdom Come Deliverance, and Red Dead Redemption 2 are very good as well. Actually, KCD is absolutely great with sound in general. Gears of War 4 & 5, as well as other Microsoft studios have put a good emphasis on their sound design also, and in those cases they even do some proper Dolby Atmos support (if you can ever find a showroom post-Covid to demo in).
If I were to recommend something though (for an upgrade) it's that I'd say go for 2.1 first, with the emphasis on a good subwoofer, as that's going to make the biggest difference and it's easy enough to add other speakers later.
Oh actually, how could I forget? Xcom 2 is absolutely phenomenal as well for surround, in fact, can easily vie for the spot as the best.