The Ukrainians do not have the tactical capability to carry out such a car bombing in Russia.
The CIA has the capability but would not in Russia.
In my opinion that makes the most likely perpetrator Russia’s FSB(successor to the KGB).
I recall that the assassination of the original Rasputin was carried out by noblemen loyal to the Czar, who wanted to get rid of a “bad influence”. History doesn’t repeat, but sometimes it rhymes.
That’s possible, buy the answers you dismiss are also possible.