I heard it was Beria, after he caught wind that Stalin was about the purge the entire Politburo.
It would probably be difficult to determine who IN FACT did it.
I think everyone was glad to see him go!
One popular version was that it was Beria and several of Stalin's inner circle. They became alarmed when Stalin calculated that since Russia had lost 20M citizens and survived that a nuclear attack on NY was worth the risk! Check into the brilliant role of warfarin in Stalin's poisoning into small amounts acre actual healthy and hence the inner circle forced to share Stalin's meals could easily consume it with Stalin in limited sittings. Beria had purchased warfarin form the US in the 1930s. The alleged tale of the inner circle torturing Beria for his murderous perversions before killing him to silence him is “inspirational”!
Hope the bastard knew it when he was on the floor