"The Sing-Off" kicked off the Christmas season with a festive holiday special (Trae Patton/NBC)The competition was over last week with the crowning of techno-wizards Pentatonix as champions, but fans of "The Sing-Off" were treated to one last night of their favorite performers when the a cappella talent show aired a special Christmas episode that brought back star contestants from all three of its seasons. The show's signature competitive tension was gone, and the seasonal spirit was apparent as the groups teamed up for feel-good collaborations.

We saw familiar faces from past seasons, including champions Nota and Committed, along with special guest Flo Rida, who brought a touch of celebrity to his performance of his hit single "Good Feeling" with Urban Method and Pentatonix. Past and present contestants brought everything to the table this week, proving that they take pride in performing even without the promise of a prize. Here are our top five moments of the holiday special:

(5) Street Corner Symphony, "Hallelujah"
Some members of Nashville's Street Corner Symphony didn't

Motown legend Smokey Robinson joined Afro-Blue on the "Sing-Off" stage (Lewis Jacobs/NBC)"The Sing-Off's" three surviving groups braved ten weeks of tense eliminations to arrive at this week's live finale, which was a heartwarming, foot-tapping a cappella extravaganza that thrust the judges, host, eliminated ensembles, and a special surprise guest into the spotlight along with our contestants. With the voting closed, the groups took a break from competing and came together for some incredible joint performances.

All the theatrics led up to the announcement we've been waiting for: Based on home viewer votes, Pentatonix, cutting-edge mix masters from Texas, are "The Sing-Off's" Season 2 champions and winners of the $200,000 prize and Sony Music recording contract!

The final four "Sing-Off" groups tackled mastermixes last night (Lewis Jacobs/NBC)For the four groups that rose to the top of "The Sing-Off's" competitive a cappella battle, tonight was the final push. Only three groups from the original 16 secured a spot in the finale, and each one is now vulnerable for elimination through America's vote (voting closes Sunday, 11/27 at 9am ET). The performers faced their trickiest task yet this week: mastermix medleys. Their challenge was to combine two hits from completely different artists into one seamless, show-stopping mash-up song. Then each vocal band returned to the stage to perform a "judges' choice" tune -- one that the judges felt would showcase their unique talents.