[Wowbagger]

Generally, I prefer one-parters by far, particularly as we get later into Trek and the cliffhangers seem more and more artificial. So, counter-posting: Shoulda Been A One-Parter:

Scorpion
Species 8472 would have been a lot scarier if they'd only been seen for a second or two, discussed only in the vaguest terms, and never infected Kim with the Martian Death Flu. All we need to establish is: the Borg are losing; Voyager can help; let's have an alliance. Do that early and then focus like a laser on the J/C conflict, which is the heart and soul of the episode and - in case you missed it - is wonderful. Oh, and Seven, too, but she's already at the center of the J/C fight, so she'll get screentime in any event.

Flesh and Blood
The argument against doing this in one part is that only in the first, slower-paced hour does Iden get fleshed out enough for the audience to understand and sympathize with him. The counter-argument is that Iden goes completely messianic crazy for no convincing reason in the second hour, so why bother with all the development in the first?

Birthright
Two disconnected stories already live here. Easily the most pointless two-parter ever.

Dark Frontier
Seven didn't even end up back with the Borg until the seventh act, did she? Interweave a slimmed-down final four acts with those (canonically preposterous but storytellingly compelling) flashback scenes on the S.S. Raven, and you've got yourself a much stronger episode.

Gambit
I usually remember this as a one-part episode, until I see an episode list.

Unification
Cut all of part one except the Picard/Sarek scene. Bam. So much better with more Nimoy by percentage.

Image in the Sand/Shadows and Symbols
The shorter this episode is, the less opportunity for the series to reveal that Ben Sisko is part-Prophet. But... maybe this isn't a fair entry on my list. Not because it's a good two-parter (it juggles too much, including that bizarre and sadly unfulfilled Benny Russell flashback), but because it may not be a two-parter at all. One could easily argue that every episode in DS9's seventh season forms a single enormous storyline that should really be taken as one contiguous multi-parter.

Shockwave
Cut Part II. Yeah, you heard me.

Storm Front
More Enterprise blowing stuff up in the sky over Manhattan. Less alien Nazis. More human Nazis. Less Daniels/Silik. (More Daniels/Silik Foe Yay.) Or, if you insist on doing it as a two-parter, please do try to make the end of the Temporal Cold War make some kind of sense, and have some kind of revelation and/or characterization that allows us to sympathize with Silik when he dies. I wanted to care; I really did!

Incidentally, on what possible grounds could anyone claim to have liked "Storm Front"? As you know, I am the opposite of an ENT-basher, and I am a huge fan of Season 4. But "Storm Front" was tremendous, boring fluff. I admit, looking back, I smile widely at the memory of Enterprise battling it out over Manhattan, but the rest of that episode just really dragged for me. So, Zeke... what gives? I'd love to love that ep if I can.