I agree that "Unification" fell sadly short. As for "The Outcast," I rather liked it, thought was well done, would have been better had Soran been played by a man. I don't think it failed the way "Unification" basically did, but it did not reach its full potential.
I actually really liked "The Chase." I think it's actually the ideal TNG episode in a lot of ways. I can see how the ending might be seen as cheesy, but I find, particularly if I'm in the right mood, that I actually find it rather touching. (Also, about the localization of the genetic key: maybe the Progenitors left 3 or 4 other keys around the galaxy so that a civilization wouldn't have to span the whole galaxy to piece it together?)
Hmm, "Allegiance"... I suppose more could have been done, but, like "The Outcast," I thought it was solid. Actually, I think it came pretty close to reaching its full potential. I liked how the ending provided a sudden and fast-moving reversal and resolution. It just seemed very tidy to me. In a good way.
In terms of the OP's original question: I find that "Descent" (parts 1 and 2) always seems to "miss" me. Data, Lore, emotions, Hugh, liberated Borg, should have been awesome. But it never really worked for me. Something just wasn't right.
"The Offspring": beautiful and poignant ending as is. (Wonderful episode otherwise, as well.) But imagine the stories we would have gotten if Lal had survived, even if just for a few more episodes. Alas, TNG could not be DS9 in this respect. (Though this isn't really the episode not living up to its potential...)
"Birthright" seems to have been missing something, too, but my memory is a little fuzzy on that one. Isn't the (really awesome) Data subplot dropped at the end of Part 1? That may have been it; just threw the rhythm of the two stories off.
Off-topic, but I think I might start a thread like this over in the DS9 forum (if it's not already there); seems to me that there are a lot more episodes from that series that fit this criterion than from TNG.
