Ukraine

Everyone (except Putin) is saying Putin.

The plane was shot down over Russian airspace with a radar jamming missile.

Also, the top three members of Wagner were all on the plane, and there was no clear line of succession. Just a standing offer from Putin to roll the mercenaries into the regular army.

The other responses are good, but I'll just add that if it was Ukraine, they would take credit for it. Assassinating a high ranking officer is a big deal, and would make them look strong. So unless there's something else going on, it's safe to rule Ukraine out.

There was no proof that it was shot done by something like a rocket. Most likely it was a bomb inside the plane. It is very difficult for Ukraine to assassinate him this way, plus, there was no benefit to kill somebody who was revolting.