Golems were way cooler AD&D. They were flat out immune to magic outside of a few specific spells. Casters got pretty creative in using the scenery and spells that affected the environment (literally affected the environment, web, grease, and such didn't cut it) when golems showed up. And honestly even the warriors didn't want to face-tank them because they hit reasonably well. Because they were ordered around you could win the encounter if you figured out the orders and hijacked them, or took out their master. If you could snipe & run until a clay or flesh golem went berserk you could get them to chase other creatures while you hid.

I always thought demons were better when they were typed and the ones in the MM were explicitly just examples of posibilities. There was a random generator in the DMG too. The whole soul-gem thing was pretty flavorful as well (or was that bit for devils? been a long time since I looked).

AD&D terrasque was better. Not so much for the stats (tho those were really nasty for AD&D) but simply because it wasn't a joke monster you could snipe or kite to death with low or mid level characters. People will argue about it, but the fact they need argue details & specific interpretations to instead of it being generally accepted the critter is a major death machine is the big strike against the later versions.