Vic Fontaine was a divisive character on DS9.  He was a holographic lounge singer who knew he was a hologram and a life coach to station residents.  Viewers seem to love or hate him.  Although he appears part way through season six, did you know the writers had been trying to add him since season four?

For the purpose of this post though, Vic is divisive in another way.  It's his premiere episode that jumps the shark for the series.  The episode is called "His Way" and it's about Vic helping Odo to finally end the awkwardness with Kira and just start dating.  On its own, not so bad, especially coming after such a fantastic episode as "In the Pale Moonlight" where Sisko teams up with Garak to get the Romulans into the war.  But the series never really reaches that height again.

In fact, I repeatedly caught myself asking "Isn't there a war on?" as we witness yet another holosuite episode (like the baseball game), or character piece about Ezri Dax.  Surely there were more stories of consequence to tell!  Wasn't DS9 on the front line??  We'd get a few mentions of some skirmish in some other sector to remind us, or unreasonable repair orders for O'Brien to fix ships returning from battle, but we'd usually only hear of this stuff in passing.  We have to be reminded there's a war on, because we so rarely see the effects of it!

In summary, Season 6, Episode 20, is where I think DS9 jumped the shark.