Taking the list from the 1969 book The Bad Popes and adding a few other choices.
Pope Stephen VI (896-897): Mutilated the corpse of his predecessor, Pope Formosus.
Pope Sergius III (904-911): Ordered murders, his papacy has been described as a “pornocracy”.
Pope John XII (955-964): A randy so-and-so who basically turned the Lateran into a brothel.
Pope Benedict IX (1032–1044, 1045, 1047–1048): The dates should tell you everything. Sold the Papacy and called “a disgrace to the chair of Peter”.
Pope Urban II (1088-1099): Responsible for the First Crusade, which started a delightful trend.
Pope Boniface VIII (1294–1303): Earn a place in Hell in Dante’s Divine Comedy.
Pope Urban VI (1378-1389): Had unruly Cardinals tortured.
Pope Pius II (1458-1464): Good buddies with Vlad the Impaler.
Pope Alexander VI (1492-1503): A Borgia famous for nepotism, debauchery and killing Ezio’s family.
Pope Leo X (1513-1521): “a spendthrift member of the Medici family who once spent 1/7 of his predecessors’ reserves on a single ceremony”.
Pope Clement VII (1523-1534): Managed to get Rome sacked, another Medici.
Pope Pius XII (1939-1958): Frequently criticised for not doing enough during the Holocaust.
Pope Benedict XVI (2005- ): Because someone would have mentioned him. Sexual abuse scandals, condoms and AIDS.
For my money, Benedict IX was a pretty obscene piece of work, Segius III is also right up there for his penchant for ordering strangulations.
Edit: Mods, meant to post this in GD - can I request a move?