I'd say the 900-day blockade from 1941-1944, when the city was surrounded by the Nazi army. But despite the death of 600,000 to 800,000 people to starvation, dehydration and cold, the city did not surrender. So it was both tragic and heroic.

In my viewshe most tragic period in the history of St.Petersburg is this that, In 1699 during the reign of Peter the Great Russia started its preparation for the war with Sweden. In 1700 the Northern War of Russia with Sweden broke out. It lasted 21 year and resolved finally the controversy of centuries. In the fall of 1702 Russian troops seized Noteburg. Peter the Great called this fortress Schluesselburg (Key Town), which marked that the route to Neva was cleared. In May 1703 Peter and Paul fortress was laid in the Zayachy Island with the church in name of Saints Peter and Paul in the middle. The house of wooden logs known as "the House of Peter the Great"was constructed on the Right Bank of the Neva River not far from the Peter and Paul fortress. May 16, 1703 is deemed to be the date of St.Petersburg foundation. In May 1704 the construction of the first sea fortress, Kronstadt, was completed in the Finnish Gulf. The location of these three fortresses outlined the borders of the future Russian Capital and its suburbs.