The fourth goal the final of Euro 2012 generated 15,358 Tweets per second

"The final match resulted in 16.5 million Tweets from fans around the world. And during today’s match, total global traffic on the platform peaked at 15,358 Tweets per second during the 4th goal, a new sports-related record on Twitter."

Source:  Twitter Blog, 1st July 2012

Earlier - 13,684 Tps for Torres' goal against Barcelona in the Champions League in April

Update - it seems that a Japanese TV show generated the highest overall tps ever, but no one really knows why:

On Dec 9, the television screening in Japan of Hayao Miyazaki’s “Castle in the Sky” led to 25,088 Tweets per second - a new Twitter record.
— Twitter Comms (@twittercomms) December 14, 2011