Excellent cartridge is the one with extended range on both ends, decent stylus profile for lowest possible record wear and superb sound quality (and very long life span over 1200 hrs), user replaceable stylus is a must have. Mid or high compliance for superb trackability.
Most of the modern MM cartridges are dull (rolled off highs is a common problem) and this is the reason why, compared to modern MM, an MC with exaggerated high is preferable for most of the users.
An MM with crystal clear treble and deep bass is very hard to find!
Some of the best vintage MM have completely different sound presentation with silky smooth highs, those carts are addictive. Hard to say why, most of them have an exotic cantilevers made of unobtainable materials (beryllium, boron pipe). Those carts are from an era when mastertape was a reference, not a digital file.
When an MM or MI is something special then today's manufacturers don’t hesitate to charge $12k for it. And for $500 they normally offer something very basic and nothing special.