My personal order of improvements giving the best sound would be;
4 the underslung counterweight which would allow for better tracking and less surface noise.
3. the 24V motor for better speed stability creating a broader soundstage and more solid imaging.
1. a machined subplatter, which while an improvement, to my ears is not as great as the first two I have mentioned.
Good luck and I have certainly enjoyed tweaking my Rega through the years.
Rega Planar 2, Incognito Rewire, Michell Technoweight, Deepgroove subplatter, ceramic bearing, 24V motor and TTPSU, Funk Firm Achroplat platter, Michael Lim top and bottom braces, 2Rega feet and one RDC cone, Grado Sonata Statement, Moon 110 LP.