Which cartridge best for RB300

A replacement would cost far more than a retip and since you like what you have a fishing expedition is more likely to be expensive than fruitful. Get it retiped, if it needs it it will sound better and you can make a more informed decision then.

Have you checked with Dynavector to see if they have a retipping program or upgrade program? I would consider that first rather than go on a blind search and buy something that you may not like at all. Just my 2 cents worth! Because your table is shielded you have many cartridges that will work well with the RB300. I have friends using Grado References, Audio Technica OC9's, VanDen Hul, Benz, Koetsu, among other cartridges that work
well with the Rega. Dynavector included. I'm using an AT OC9MLII with very good results. It's not in the same league as the Dynavector XX but it is better than it has any right to be.
The biggest benefit to my friends RB300's was to have the arm rewired by Incognito which raised the arm to an all new level. I'm trying to determine if I want to have my RB600 rewired. Some of my friends say yes, and some say no. It also widens the types of cartridges that could be used also. You can use MM, MC (Lo or HO), and MI designs. You have great gear and if the sound you are getting is great talk to Dynavector first for their offerings.

Ponnie brings up good points... if you plan to keep the RB300, you'll get better performance with some modest upgrades like rewire, VTA sleeve and Michell Tecnoweight. Personally, I found the Tecnoweight offered the most noticeable improvement.