BEST TONEARM FOR HIGH COMPLIANCE MM

I'll second the Black Widow recommendation, and add the SME 3009 III - those two are my favourite low-mass arms.

other optional arms I like are the Grace 707 and the Mission 774 LC, but the former toe are distinctly better.

The XSV4000 isn't really a high compliance cart. Cu should be similar to an Ortofon 2M. I'm not exactly sure what the Pickering is, but my Stanton 980 has the same VTF and cu is 20.

The Stanton tracks like a banchee on med/light arms and I doubt if you'll have a problem with a reasonable match. I like it on my Unitrac w/original wiring that works just fine, but it should be okay on an arm in the 11 or 12g range.

Back in the day high compliance carts were up to 50cu. IMO, with similar cart the Unitrac bests the other arms mentioned.
Regards,

Grace 707 is a fine arm for a high compliance cartridge. A step up from the Grace is the SME 3009/3012 line.

How about new Well Tempered Lab LTD MK2 tonearf with effective mass of 10g (for very high compliance cartridges) ?

I am using a Signet XK50 with the Soundsmith Aida High Compliance Cartridge. The Signet was one of if not the first to use the counterweight so that it was on line with the stylus thus providing even and constant downward stylus pressure on the varying surfaces of vinyl. The cartridge tracks at 1 gram.