In absolute numbers, China, which is not a big surprise, given its huge population:
If we instead look at the issue taking into account the population size of each country, we get quite a different ranking:
Source: I divided the executions reported in the first graph by the population of each state, the figures of which I retrieved on the English Wikipedia.
Iran is first with 1.23 executions per 100000 inhabitants, followed by Saudi Arabia with 0.51. All the other countries lag behind.