Most probably Kirov. If Stalin lived long enough to nominate a successor (and his nomination had weight), possibly Zhdanov, Andreyev or Malenkov (though he might be thought a bit young). If Stalin's death caused something of a reaction against him, Bukharin. If support of the Army proved necessary, Gamarnik would have had a good chance. Rykov might have made a comeback and Litvinov could have been a compromise candidate in event of a deadlock between any of the preceding.
IMO Kirov's importance was magnified by his assassination, and he had not really stood much chance of being Stalin's successor. The best work on Kirov is Matthew Lenoe's *The Kirov Murder and Soviet History* (the basic thesis of which is that Nikolaev probably acted alone in killing Kirov). Two points Lenoe makes:
(1) One should not exaggerate Kirov's stature in 1934. "Not only did the early publications of the memorial campaign turn Kirov into a plaster saint, they also exaggerated his stature in the party leadership. Perusal of *Pravda* and even Leningrad's hometown *Leningradkaya pravda* from 1934 suggests that Kirov's public profile before his death was substantially lower than that of Kaganovich, Molotov, or Ordzhonikidze. Coverage of the Leningrad leader was comparable to that of Pavel Postyshev and other second-level party officials. The overstatement of Kirov's power and prestige during the memorial campaign contributed to later assertions that he was a serious rival to Stalin." *The Kirov Murder and Soviet History,* pp. 494-495.
(2) Lenoe also argues that it is not true that Kirov got more votes than Stalin for re-election to the Central Committee at the Seventeenth Party Congress: "I. F. Kodatsky, from Leningrad, and Mikhail Kalinin were the only two TsK members elected unanimously. Stalin received three votes against, and Kirov four." p. 757. There were allegations in 1960-61 that there had really been two or three hundred votes against Stalin, but Lenoe dismisses them as implausible and designed to fit Khrushchev's narrative of the time--that many "honest Leninists" had tried to stand up to Stalin, but were thwarted by Stalin, Molotov, and Kaganovich--the last named was very conveniently accused of altering the voting results. pp. 613-614.) Of course that there were any votes at all against any of the leaders--especially Stalin--was suppressed, and in later memory that may have been inflated (or deliberately exaggerated) into suppression of a huge number of anti-Stalin votes.
IMO Molotov has the best chance. He was respected as a hard worker. Not imaginative, but it is doubtful that the Politburo wanted someone imaginative at the time. Andreyev, though a full Politburo member since 1932, was not of the same stature. Zhdanov was not even a candidate member of the Politburo until 1934. It's way too early for Malenkov. Kaganovich's Jewish origins would probably be enough to disqualify him. Former "Right Oppositionists" like Bukharin and Rykov would have a better chance of surviving than if Stalin had lived, but I can't see them returning to the really powerful offices. Former "Left Oppositionists" had even less chance of a return to power.
BTW, a Stalin death in 1933 is quite plausible. "...in August-September 1933, he [Stalin] was involved in two accidents, both of them potentially fatal. In the first, his automobile nearly collided head-on with a truck on a dark road outside Sochi. In the second a border detachment fired on his motorboat by accident near Gagra on the Black Sea. Stalin ordered measures taken to investigate each incident and prevent anything similar in the future, but he did not treat either as a potential assassination plot..."
https://books.google.com/books?id=VTyQA8z-XQ4C&pg=PT684 We tend to forget that even in Stalin's USSR, sometimes people (including people whose deaths would be convenient to others) really did die by accident...