What is the "best" cartridge a Technics 1210...

First you have to convert Denon’s (DL-301 MK2) compliance from 100Hz (Japanese only system) to 10Hz (Western world system):

Compliance of your Denon is 13x 10-6cm/dyne @ 100 Hz
You have to multiply it roughly on 1,7 to find out what is the real compliance of your cartridge @ 10 Hz

13 x 1.7 = 22,1 cu - this is real compliance of your cartridge.

Then you have to count not only effective mass of the tonearm + headshel (which is 12g as you said), but the mass of the cartridge itself (which is 6g). You have to count in the lead wires weight and screws/nuts weight too.

Then USE THIS ortofon diagram to calculate resonance frequency.

You can also use Hi-Fi Test LP to calculate resonance frequensy by playin this record with your actual cartridge/tonearm.

As you can see the resonant frequency with stock headshell and your Denon will be around 8Hz (roughly). Resonance frequency within 7-12Hz is optimal for the system! Technics stock headshell is a good one if you have already replaced stock headshell lead wires. Before upgrading the stock shell you have to completely rewire your technics with Discovery Tonearm Wire (internally) and with external RCA phono cable of your choise. 

I think you need lighter headshell, not heavier if you are so concerned about resonance frequency with this tonearm/cartridge combo. Or you can simply ignore this theoretical point of view if you play flat (not warped) records with heavier headshell.

P.S. I have 8.4g DENON headshell for sale here. You can also try the Orsonic lightweight headshell.

@chakster Nice post, good advice. 
@lordvampire , "all the good headshells I see are 12g". No. Yamamoto makes a number of lighter ones including 7g Boxwood. Cheers,
Spencer

@chakster  Hi, there, thanks for the perfect reply.
I made the calculations, and actually it turned out that for the resonant point of view, the headshell should be even less than 6g, in order to match the perfect 10Hz resonant frequency.
But I also read that MC cartriges need higher mass headshells, for some reason.
Maybe the original technics headshell is perfect after all, but I will have to change its wires, because they are very basic. Also, I'm not sure about it's material, from another forum I read MC should Not be used with headshells that have anything metal on them. 

About Discovery Tonearm Wire .. Is this the best choice for technics stock arm? Or at least is it good enough for me, after all I'm not gonna use it with $1k+ cartriges after all. But it seem cheap, for such an important thing as the wires for the tonearm.

And one more question. I can hear the power supply when I turn maximum gain on anything. I know it's the power supply because when I turn off the turntable this low noise stops.. 
Will the achromat mat help to reduce it? Or maybe better feet?
I still don't want to touch the original technics power supply.

http://www.needledoctor.com/Denon-DL-103R-Cartridge
It looks like this cartrige makes perfect match with Technics stock tonearm, when used with 12gr headshells which are easier to find :)
Maybe I should rethink my choise, but from what I read 301 is more warm, which is something I really like... But actually I'm not sure because it is so subjective, does anyone listen to both cartriges (103R and 301 MK2) to tell what are there strengths and weaknesses.