Best author:
Michael Stackpole. Yes, everyone will keep telling me I am insane, and I will keep telling them
I, Jedi was awesome. (And, incidentally, it fixed several leftover plotholes from the KJA Jedi Academy books.) It could be called "deeper" writing than most other EU, although this is a highly relative qualifier, and is certainly more entertaining than most.
His
X-Wing books were not awesome, but not exactly awful either.
After him, Allston's works are rather good, for all the reasons others have mentioned.
(Both of these authors belong the extreme end of the school of what was known as "minimalism" on this board, back when people still cared about the VS Debate. I am, however, leaving that out of my assessment, here, and judging them merely by their literary merits.)
Matthew Stover wrote an excellent novelisation of the last of the prequels, easily one of the best SW works, period. Vastly better than the film itself. On the other hand, he also wrote the awful
Traitor, so his authorship seems to be somewhat hit-and-miss. Or perhaps he was not responsible for most of the plot of that one, seeing how the NJO was (supposedly, at least) written as a collaboration.
Brian Daley's Han Solo books had their moments of awesome, and were more typical poorly defined "soft sci-fi" (in a good way, or so I think) than the rest of the EU. Being very early stories, they are closer to the original film (
Star Wars, later also labelled
Episode IV: A New Hope) than the EU, or indeed in some ways the sequels themselves. Swashbuckling adventure for the win.
I will add the much-hated (?) Dave Wolverton to this list also. I can understand perfectly why some people dislike
The Courtship of Princess Leia thematically, but I found it excellent.
Interestingly, comic books generally seem better than novels.
Dark Empire (not, for the most part, its sequels) and much of
Tales of the Jedi are often awesome, as well.
Then, other authors were more average, but not bad.
Kube-McDowell is probably my favourite among these, although he suffers from some EU brainbugs. About half of his books were total crap, but those concerned with the "main plotline" were fairly good, and he shows that he actually did some research and has some knowledge about what he is talking about when he describes militaries. (Less so with some other things, but whatever.)
Timothy Zahn's early books are technically good writing as such, but his cluelessness about political and military matters irritates me. (Unrelated to his famous minimalism, in this case; rather, realism issues on how these kinds of organisations work and interact.) Then again, he is hugely praised by others, so perhaps I hold too high a standard in this regard.
His later works tend increasingly towards Mary Sue of/for his pet characters, tendencies of which were already apparent earlier, but forgiveable.
Anne Crispin writes tolerably, some stupid plot devices aside, but caricatures the Empire as dysfunctional super-Nazis. Then again, that is not exactly uncommon in the EU.
As for really bad authors, it would be the usual list already supplied. I would, however, add the infamous (?) Lando Calrissian trilogy by L. Neil Smith. It reads like it was written on drugs, introduces some of the most ridiculous stupidities in the EU (Waru is fairly benign by comparison), and - for me, at least - just feels unreal when read.
As a concluding commentary, I must bewail the lost potential in the "Jedi Prince" books which Eleas referred to. If only those plot lines had not been written out for eight-year-olds, but taken seriously, they would have been utterly awesome. Imagine the visions: A brilliant, charismatic, Force-sensitive evil prince of the Empire - an anti-Luke, essentially - chasing Leia's heart. An insane son of Palpatine locked up in a hidden asylum on a quarantined world, dreaming dark dreams. Luke finding old Jedi relics. An order of Dark Side magi secretly conspiring to assume power, then launching a grand crusade to purify the galaxy, led by a visionary prophet.
Instead it was plothax KJA could only dream about with an utterly vile Gary Sue ten-year-old "Jedi Prince" as the main character, Imperials who make anime villains look smart, and all the OT characters dumbed down to this level.
