MC cartridge that best mates with RB300 arm....

I second the Dynavector recommendation. Absolutely musical in an RB 300. Available in high & low outputs - but some say the low outputs sound a little better, if you have the correct preamp for them. I have no real experience in that, I've only used the low outputs, and have been very pleased. Dynavector is one of the true bargains in Hi-fi. Check out the info at:
http://dynavector-systems.com/dynavector/
A 20X-H or 20X-L should be magical for you.
Good Luck!
Ed

The dynavector 20X-H or -L, depending on your phono stage. The lower output MC's can be very demanding on the front end, and on the tt/arm. The 20x-H should work on a MM front end, due to its high output. I am not sure a the rega 3 and 300 arm can handle a low output without an Origin modification to the 300 arm.

AT OC-9.

If you go DV-20x, get the high output. Getting the LO is pointless with the P3/300, and a bit risky, sonically. Personally i'd get the 10x4, and possibly put the rest towards a Heavyweight or save it to partially pay for a TT or wire upgrade. Good luck.

The 10X4 Dynavector is spectacular. Don't forget a 2mm spacer to adjust the VTA with a Planar 3. I've tested it with a Cardas and an old Shure 117 test record. It sounds great, skates perfectly and tracks almost as well as an old V15Shure cartridge which could track warped records that looked like a washboard.