The Threadripper 3990X from an unexpected perspective: cheapest operational costs per thread:

And the 65 Watt tdp 12-core Ryzen 9 3900 (so without 'X') has the most L3 cache per core/thread, while staying close to the lowest tdp levels per core/thread. The hypothetical 48-core Threadripper 3980X would be of equal level. The 'X' model of course has the same L3 values.
Note that the Intel SOCs and CPUs all have IGPs here, just as the Athlons and the Ryzen 5's (that of the Pentium G2120T isn't useable in BOINC though).