Since VLR, RIB, and THESPIKE all have ratings, but they are all really complicated (I know VLR's is because I designed it, and RIB's is open source), I made a "reverse engineered" approximation using only kills, deaths, assists, first kills, and first deaths.
Some TLDR notes:
I can get under 2% relative error for THESPIKE while VLR and RIB are too complex to approximate with my methodology.
THESPIKE values kills ~10 times more than deaths
The value of an assist relative to the average kill/death is 0.77 for THESPIKE, 0.43 for VLR and 0.32 for RIB
RIB heavily favors opening duels (particularly deaths), but this might be somewhat affected by the poor approximation for VLR/RIB.