best MM cartridge...

Since Raul listed B&O MMC2s moving iron cartridges, how about SoundSmith's modified B&Os?

Try a Dynavector DV20XH - a MC that can be used with a MM phono stage. I use one on my Rega P7 with the phono stage on my Rotel RC-1090 pre.

No one spoke about the Adikt?
However, sticking with what you have for a frame of reference is a good chunk of advice.
(A sub 300 dollar cart will usually sound better in a 800 table than a sub 300 dollar table with an 800 dollar cart.)

Dear Jlamb: Right now there is a rare opportunity, you can buy an almost impossible to find MM cartridge ( one of the best quality performer ever made ): Audio Technica ATML-170 OCC in NEW condition ( boxed ) through ebay Canada.
This cartridge do justice to your title thread.

Regards and enjoy the music.
Raul.

I've used a Stanton 981 HZS for over 25 years,my table is a Thorens 147 Jubilee, and have spent quite a few bucks with both MM and MC trying to best it. By far the best investment I've ever made in my system.