It appears that in many years the Superbowl is indeed the most watched event on TV - certainly the most watched sporting event. It is only outdone by the soccer World Cup and by Olympic opening and closing ceremonies, which don't happen every year.
The FA Cup, as far as I can gather, is nowhere near Superbowl standard. The UEFA Champions' League final is starting to run the Superbowl close, but hasn't got their yet. Superbowls hover around the 95m mark. The Liverpool-Milan final in 2005 was watched by 73m.
F1 Grand Prix sometimes break the 50m barrier, and they are the next most-popular annual events.
Caveats on this.
Around 80% of the Superbowl audience is in the US. It has numbers, but it is not globally popular the way F1 and soccer are, nor is it likely to increase much as the NFL's attempts at selling the game outside the US have been a damp squib everywhere except in Germany.
There may be sporting events in China that attract bigger audiences that we don't know about. I've heard it suggested that over 100m Chinese will watch a Premiership match if there is a Chinese player in it, but data on Chinese viewing can be unreliable. Then again, we do know that viewership in China is growing rapidly.
When you include non-annual events, the Superbowl gets blown away.
The audience for the 2002 World Cup Final was at least 250m (though nowhere near FIFA's claimed 1.1 billion). 2006 was somewhat smaller, probably due to the early exit of Brazil.
When people in India start getting more TVs the Cricket World Cup will far exceed Superbowl as well. Currently only about 125m homes in India have a TV, but the population is 1.2bn. Around a third of all Americans watch the Superbowl. Given that Indians are much more passionate about cricket than Americans are about gridiron, we could expect an audience of over 400m for a world cup final featuring India.
Some of the data sources:
Super Bowl Viewing Figures
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