Natasi Daala: The Best Admiral We Never Had
Daala wasn't very well-written, but we can logically infer that she must've been a talented leader. She is the only one to see the fundamental problem the infighting Imperial warlords represent, does her best to solve it, is in fact reasonable about it, and ultimately does what must be done -- and
succeeds. In spite of comple foolishness on the part of those warlords, she sill united the remaining Imperials. And within a short span of time, not only gets re-armament started, but successfully introduces reforms that end the institutional sexism and bigotry (which needlessly limited the appeal of the Remnant to most non-humans and women).
And then she proceeds on a campaign that is generally successful, has clear tactical objectives (draw away the Republic fleet, then attacks the Jedi, because they represent an existential threat), and which is only foiled by something she literally could not have known (namely that Jedi can combine their powers and literally push an entire fleet several astronomical units away in an instant).
Overall, I say that she's severely under-rated because she was poorly written. Her actual performance is actually comparable to Thrawn's. She creates the Imperial Remnant by sheer forces of will, which just so happens to be the one Imperial successor state that manages to stabilise and survive. Similarly, Thrawn created the Empire of the Hand. Likewise, they are both undone by factors they couldn't very reasonably predict, stemming from knowledge to which they had no access (Daala couldn't have known Jedi could do that specific thing, and Thrawn had no way of knowing Luke and Leia were Vader's children and therefore had a connection to the Noghri.)
Disagree on Daala. We never got Gilad Pellaeon.
I'd say Pellaeon was a very capable administrator, but not actually such a skilled admiral. Nor did he have particularly stellar leadership credentials. Note that before Daala appeared, he was unable to pull off what she did. He saw the problems with the warlords, but wasn't able to solve those problems.
Pellaeon was an excellent second-in-command, and a fully qualified successor to a figure like Thrawn or Daala. Level-headed, dependable and a man of his word. And once in charge, he grew into his role for sure. But without someone else setting things in motion, I don't see Pellaeon pulling such achievements off on his own.
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Mara Jade: The Only Way to Make Luke's Exile Understandable
I don't think it's the only way. The idea of his nephew turning bad (and Luke outright standing over his bed with his light-sabre ignited) is what screws it up. The weird attempts at retcons ("no, Ben didn't
really kill the other students!") only make it worse.
But consider the following: from what we know about Lucas's plans, some kind of tragedy with a student betraying Luke was always the plan. But it wasn't supposed to be his nephew. Lucas, at least at one point, envisioned an expy of Darth Talon in the role. We can easily imagine a plot like this. Post-RotJ, Luke aims to rebuild the Order and gahers students. One of them (the Darth Talon-to-be) is clearly drawn to the Dark Side, but Luke is confident he can guide her to the Light. He redeemed Darth Vader, after all! And he believes in the fundamental good in people!
What follows is something akin to what repeatedly happened (and really, too often) in the old EU. As with Kyp Durron, or Brakiss. Luke is
wrong, and his student falls to the Dark Side. And in this case, that student ends up murdering all his other students. And then disappears into the night. Luke is left with a bunch of corpses, of the young people he was entrusted to teach and protect, and he feels that it's all his fault.
With that kind of set-up, his exile makes sense. He feels like a failure. He got people killed. Instead of whining about the supposed arrogance of the prequel-era Jedi, he laments his
own arrogance. He thought he could easily save anyone struggling with the Dark Side, and as it turns out... he couldn't.
(And then another young woman shows up, with a lot of buried resentment, who reminds him so very much of the one who butchered all her classmates years ago. If well-written, his reluctance to train her as a Jedi makes perfect sense, too.)