Depend on who take over:
Bukharin, Rykov, Kamenev, Tomsky, Zinoviev, Sokolnikov, Kirov, Trotsky (probably worst case scenario), Frunze, Voroshilov, Molotov, Smirnov (although his chances are very slim because he never made it to the politiburo)?
Remember all of them are different personalities with different ideas how Communism work, so they will all be very different in power. If you want worse than Stalin, Trotsky is the best choice, with his "collectivisation now" and "USSR doesn't need a foreign policy" ideas. Thankfully Trotsky was quite despised by the Party, so his chances were always small.
Best case, maybe someone from the right wing of the party like Bukharin, Rykov or Tomsky. At least they were not that into getting rid of NEP, although NEP would probably have to go sooner or later no matter who is in charge.