best cartridge for MMF-7, stock tonearm,below1000

If you wanted to stay under $1000, the best play might be to sell your entire rig, add $1000, and buy a new (used) rig. for $1500-2000 you can get a very good deck - I've seen LP12, Scout, various Sota and VPI HW, Nottingham, etc in that range - all better basic decks than the Music Hall/Pro-ject. Heck, I recently got an Oracle/Koetsu for that.

I believe Inscrutable has the best suggestion. The Rega arm is not a drop in replacement by any means as there is too much difference in the spindle to pivot distance. You could look for a Linn-based tonearm to upgrade with, but since you seem to want to improve things I think in the long run you will be happier swapping out the table now.

Radiob,

Inscrutable & Dan_ed got here before me & have already echoed my sentiments.
The cartridges that you are looking at as replacements are rather resolving in nature & are capable of some really nice dynamics & music BUT on a TT rig that is able to provide them the opportunity. Bluntly put, the MMF-7 is a good TT rig but it is not very resolving one & the isolation is mediocre (very good for it's price range, mind you, but not in the league of SOTA or Nottingham or VPI Scout Master or an upgraded Linn LP12) i.e. even if you are able to keep the MMF-7 surroundings as quiet as you possibly can, the MMF-7 is limited in its inherent design. Thus, the Shelter 501, Dynavector karat, Zyx R-100 will provide you marginal improvement in sound & you would have spent a lot of money for a marginal improvement (low ROI). The Pro-ject 9 tonearm is good - the best Pro-ject tonearm, but it doesn't hold a candle to a stock RB250 & definitely not to an OL 250 or an Expressimo RB250. I don't know what the eff. mass of the Pro-ject 9 is. The Shelter 501 is a very stiff cart (low compliance) & will need a medium-heavy mass tonearm. The Pro-ject 9 might not qualify. Also, the Pro-ject 9 is prone to bearing chatter over time just 'cuz of its bearing design. Worse, it is very likely that you cannot use the RB250 as a drop-in replacement for the stock MMF-7 tonearm! It was never engineered to be a TT rig that was upgradable as Roy Hall wanted to make a good plug-and-play TT for those that didn't want to go thru the pains of vinyl setup & tweaking.
If you notice the Pro-ject 9 tonearm headshell, you'll see that it has just 2 holes in the headshell where the screws for the factory fitted cart is mounted. Are you sure that this fixed locations of these screw holes will allow you to mount 1 of the 3 cartridges you are considering? It might be an expensive risk on your part! Look @ your RB250 headshell - it'll have slots which'll allow you to mount practically any cartridge (most likely barring the Dyna 17D Mk2 as it's cantilever is miniscule & you'll need even longer slots!)
It appears that your appetite for vinyl was been whetted by this product (& mine was too when I had the MMF-5). So, if you remain serious, like Dan_ed, I say that Inscrutable has metted out good advice.
Hope that this clarifies some. FWIW.

Even a broken watch gets it right twice a day ... but only once if it's digital ;-)

thanks for all of your guys advise.Bombaywalla,you have been a great help, this was excactly the information I was hoping to find, I only wish the dealer I purchased it from was so straight with facts. I also have a rega planar 2,I decided 2 list them both, and upgrade two a better deck. I set a budget of 2500.00 any sugestions? Thanks, Rob