1984 wins hands-down because it was simply an extension of what was already happening in the Soviet Union under Stalin at the time. The doublespeak, the personality cult, the fawning adoration, the bizarre puritanism that one wouldn't necessarily expect from an atheist state - it was all there.

Furthermore, the world was subjected, in 1937 and 1938, to show trial after show trial where the accused admitted their guilt in open court. People in the West didn't realize that several "fake" trials were conducted beforehand (complete with fake foreign journalists) to see if the accused would remain frightened enough to actually go through with the (false) confession (they usually threatened killing the whole family, after torture, including distant relatives).

Given that China went communist in 1949, all of Eastern Europe was under Stalin's control and communist movements were springing up all over Latin America, Africa and Asia, when 1984 was written, it was a very realistic (and chilling) possibility for the world. In fact, it wasn't (ironically) until the Soviet Union bankrupted itself in the 1980s that the threat of a 1984 world faded from memory.

Brave New World is the sort of dystopia that one would expect from a free, permissive society, and so it remains something to be concerned about even now that the Marxist threat has disappeared.

Of course, North Korea still looks like 1984, and Cuba isn't far off, and Hugo Chavez is trying to turn Venezuela into a workers' "paradise". Of course, in all three examples they'll bankrupt themselves and destroy their economies much faster than it took for the Soviet Union to collapse (North Korea has no economy, effectively, and Cuba's is defunct, with Venezuela heading there fast despite its oil reserves).