I mostly gamed on the mac from 1986-2002.
Almost everything I can play elsewhere now except:
Marathon runs fine in modern OSes with Aleph One (software rendering with low resolution mode enabled in advanced options for the original two games though please!) but the still incredibly well-designed editors Forge and Anvil still only runs in Classic.
Shadowgate’s original black and white mac version is the definitive one for me, but the NES port and recent remake are good in their own ways.
all the Maxis Sim games from the 80s and early 90s are best on mac, especially the original Black and White Sim City and Sim City 2000.
Solarian II is a good Galaxian type game
Crystal Quest with its Critter Editor especially!
The 1988 Macintosh Tetris is the definitive Tetris to me, a cool thing about this era is the Mac was the only platform where everybody had at least 512x384 and everyone with color had 640x480 so many games had high res art exclusively in the mac version.
the Macintosh release of Lode Runner is my favorite original version.
Dark Castle, Beyond Dark Castle, and Return to Dark Castle are classics if extremely frustrating ones.
Snakes Alive! is still my favorite snake game
Bubble Trouble is the best Pengo clone, better than the original
Ferazel’s Wand is a fun action platformer you can only play with a real Classic mac, last I checked at least no emulators supported some of the PowerPC extensions it relied on.
(those two are Ambrosia Software, who were accepting money for their games on their website but not sending out the actual license codes last I checked so be wary.)
Return to Zork (lots more full motion video than the DOS version the rereleases are based on)
Voyager CD-ROM releases, the precursor to the Criterion Collection, are a fascinating treasure trove.
The Macintosh was the best place for almost all the multimedia/FMV edutainment/games CD-ROMs of the early to mid 90s. Honestly anything released for Windows 3.1 and Mac I would recommend running on a Mac.
I also keep old Macintoshes around to run my favorite MIDI sequencers and synth editor, Opcode’s Vision, Musicshop, and Galaxy. I’ve used dozens of modern DAWs and still none of them do some of the things I enjoyed about these.
KPT Bryce 1/2/3, Photoshop 3/4/5/6, and Kai’s Power Tools are other classic mac software I wouldn’t recommend running in Windows.