The Entire Original Series, because it started the entire thing, but most of all...
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Star Trek: TNG
Picard surrendering 21 minuets into the pilot episode... setting the pace for what a pussy he'd be throughout the series, The Enterprise C, anything with Q, and all the appearances by the original series alumni.
Star Trek: DS9
Trials and Triblations, the 3 Klingons from the original series, The Dominion War, The Mirror Universe, and Sisco punching Q.
Star Trek: Voyager
Lost is Space theme, The Year in Hell, finding the lost Ferengi from DS9, Q, Q's son, Janeway as Ripley, 7 of 9.
Star Trek Enterprise
Season 3, Mirror Universe, When Niles and Trip were stuck in the shuttle together, the decedents from the Alternate Future.