Multiple realities
(covers information from several alternate timelines)
"Duty. A starship captain's life is filled with solemn duty. I have commanded men in battle. I have negotiated peace treaties between implacable enemies. I have represented the Federation in first contact with 27 alien species. But none of this compares with my solemn duty best man!"
– Jean-Luc Picard, 2379 (Star Trek Nemesis)

In traditional Human wedding ceremonies, the best man was the male assistant to the bridegroom. The Klingon equivalent was known as the Tawi'Yan.

Robert Tomlinson's best man was an operations division crewman during the aborted wedding to Angela Martine. (TOS: "Balance of Terror")

Geordi La Forge was best man at Keiko Ishikawa and Miles O'Brien's wedding in 2367. (TNG: "Data's Day")

Alexander Rozhenko served as Worf's Tawi'Yan in his marriage to Jadzia in 2374. (DS9: "You Are Cordially Invited")

Benjamin Sisko chose his son Jake to be his best man in his wedding to Kasidy Yates in late 2375. Jake looked forward to planning the bachelor party. (DS9: "Penumbra")

In an alternate timeline, Harry Kim was the best man at the wedding of Tom Paris and Kes. (VOY: "Before and After")

Jean-Luc Picard was given the honor being William T. Riker's best man to his marriage to Deanna Troi in 2379. Picard, who had served as commanding officer to Riker and Troi since the maiden voyage of the USS Enterprise-D, was proud and honored to take the title of best man. It is a tradition for the best man to give a speech celebrating the newly married couple, and Captain Picard did just that; his heartfelt speech to the happy couple was admired by all in attendance, most notably Deanna. (Star Trek Nemesis)

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