A case could be made that it doesn't count as a main release, but I think it does.
I'm sure there are plenty of people having a lot of fun with it. I know Hoard of the Dragon Queen and Dragon Heist have worked for me and mine, though the criticisms in this thread are quite valid.
Best: Players Handbook.
Other than the free version of the basic rules, it's the essential book of the edition. Everyone at the table, Player or DM, needs a copy. Everything else in the edition hinges upon this book and it sets how the edition is judged/accepted.
For the most part it does a great job.
Worst?
I'm not going to judge the books I haven't read - Ravnica, Ebberron, & Acquisitions. Just because the content isn't of interest to me doesn't make it bad....
So my vote goes to Sword Coast Adventures.
Why? Because it's just been pretty much useless except for a few select pages in our games.
I agree about the PHB - both great, and terribly frustrating.
Aside from PHB, my favourite would be Xanathar's, it has better DM stuff than the DMG, including very nice tiered encounter tables.
Not sure about worst - I disliked Lost Mine of Phandelver but I will try running it again soon, I reckon I know the reasons it didn't work for me first time, primarily the beginning and the quest-hub NPCs, and those can be dealt with. I bought HoTD & RoT recently but haven't read most of them. Adventures in Wilderland for AiME was terribly ralroady, but not really 5e I guess.
For "best" it's difficult to evaluate together core and supplements. For sure, the PHB is the best and more valuable book because it's the GAME itself actually.
On the other hand if you ask me what's my favourite (not the same question) I'd say the MM because that's what I still enjoy reading most even casually, and it's my favourite MM across all editions.
Among non-core books it would be XGtE, thanks to the fact that it complements (or even complete, in terms of subclasses) the PHB nicely. I really don't feel I need anymore character material after XGtE.
The worst is easily SCAG. It's way too narrow to work as a setting book for me, it feels more like a demo. And it's very light on character material too. I dislike the focus on backgrounds because those are the easiest material to do by yourself (they literally require no playtesting). And the best subclasses were reprinted in XGtE, making SCAG even less valuable. The only way to redeem the book would be to make it part of a larger series of regional sourcebooks, which doesn't seem to be WotC plan.
The PHB. Organizational issues aside (and they certainly exist), it introduced a great new edition. One that manages to both feel modern and yet channel the older editions of the game. Two mechanics particularly stand out:
1. Concentration: one of the best balancing mechanics to allow spell casters to do their thing but not completely outshine non casters.
2. Advantage/disadvantage: a simple yet elegant mechanic both for combat and out.
Worst:
I'm going with Waterdeep: Dragon Heist. As many problems as HoTDQ has, it doesn't rise to Dragon Heist.
1. The "heist" isn't one performed by the players. If you advertise a heist, give the players a heist!
2. The adventure has too many instances where it forces you too wander around for no good reason until you stumble upon the path to the next section. It's meant to appear open ended, but it really, really isn't. Often, that's worse than admitting it's linear.
3. A 60 page section on villain lairs that the book itself admits, if the players actually explore these lairs, they're likely going to suffer a TPK. I get making the adventure feel more real etc, but 60 pages that doesn't actually fit the adventure is too much.
3. The single worst example of railroading I've seen in an adventure, and I've recently re-looked at the Dragonlance modules!
the mcguffin itself will literally force the players to forget they found it, if the DM thinks they found it to early!
That's not clever or cute, it's preposterous and horrible.
Favourite = PHB. It constitutes the core gameplay, but more remarkably it advances the game in ways informed by the design arc of Book of Nine Swords and 4e. The mechanics achieve a level of sophistication that is transparent to readers, pleasing players across the spectrum.
Least favourite = SCAG. I think the crunch is SCAG is overall poor, and questionable when it comes to balance. The fluff, given the wealth of exceptional material from previous editions to draw on, is badly organised and fails to shine. It's the only book I have that I haven't gotten value from. Possibly they wanted to leave room for the Faerun material they wanted to publish in modules such as SKT? I like WotC's updated approach to those modules (some world material, some crunch, and the main adventure), but they might have served players better by publishing the SCAG material across those modules.
Best: the PHB and DMG. I like the character creation process, reduced skill list, easier combat as well as the new mechanics introduced. Overall I think the core system is a great balance of taking everything that was good in the various older editions to make a fresh system. I love that 5E is somewhat vague at times and put the power back into the hands of the DM. 3.x had every rule so quantified that players could exploit that.
Worst: Any DM screen released for 5E. Ive never even used one during a game, thats how useless Ive found them. XGtE is a close second for me. Reading and skimming it, besides translating the material to 5E mechanics most of it Ive seen printed in one form or another over the various previous editions of the game.
