Breakdown of Turntable Performance

Briggs and Stratton - VERY good, Uberw.

The requirements for the turntable depend on quality of the cartridge it has to serve.  I was very happy with my at-lp1240 til I had my AKG P8ES Supernova upgraded with a boron cantilever.  The upgrade made a huge difference in responsiveness, but the byproduct was the direct drive motor noise became much more noticeable once the stylus hit the vinyl.  It wasn't a problem for me before.  I'm now looking for a belt driven turntable so I can enjoy the cartridge more.

My brain can only grasp what sounds good to my ears.  I have a 30+ year old Technics SL-1600MK2 with Ortofon Blue in my rec room and a one year old Technics SL-1200G with Ortofon Quintet Black in my listening room.

At first, I had the Blue on the 1200G.  It sounded pretty good.  I then bought the Black.  All other things being equal (record, phono preamp, etc.), I'm sure I don't have to tell you which one sounds waaaaaay better.

All this turntable talk and no love for the Well Tempered concept?
Chatter-free viscous damped platter bearing, chatter-free viscous damped tonearm bearing, physically isolated motor?  WT has 'em all.  I bought the original WTT/TA in 1988 after owning an LP12, a Technics SP-12 w/AT tonearm, and a SOTA Sapphire with a Premier FT3.  I found it to be superior to all of them in quiet no-stress sound and operation, incredibly static-free in the dry months, and have never even thought of replacing it.  I now run a vintage Grace F9 Ruby with low hours in it after using a van Den Hul MC10 for most of its life.  Look into their Amadeus.

The OP asked about relative IMPORTANCE, and most guys responded with a COST analysis.  That's just one reason why the question is so complex. On a cost basis, I have often found that a very fine turntable and exceptional tonearm can bring out unexpected qualities of a relatively inexpensive cartridge, but that's where the complexity of the question enters into it.  If the inexpensive cartridge can sound so good, it is also an indication of how poor is the correlation between cost and performance, for phono cartridges.  If you leave out cost, all 3 components have to work well together, else you will never get the most out of the cartridge.  So, I don't know how anyone can say that the cartridge is vastly more important than the tonearm and turntable. (The OP says "80%" for the cartridge. Some others estimate an even higher percentage contribution from the cartridge.)  Cartridges fall right on their faces without the contributions from turntable and tonearm.