Aquaman Vol. 4: Death of a King might be the greatest. Johns uses an obscure villain here (at least "obscure" to me), enough so that until I looked it up, I thought Johns was charting his own path, and the book is better for it -- it feels fresher, as if more focused on telling a story than proving Aquaman's "worth." Combine this with some genuine surprises and the result is a solid book.
[Review contains spoilers]
If you are like me, then you thought the Scavenger was a skinny old man that used to tangle with Karl Kesel's Superboy. If you are still like me, then you might have been pretty impressed that Johns repurposed Scavenger in the New 52 as a corrupt treasure hunter that tangles with Aquaman and just so happens to wear a diving suit that looks like an open-mouthed shark. In Aquaman Vol. 2: The Others and Aquaman Vol. 3: Throne of Atlantis, Johns spotlighted Aquaman foes Black Manta and Ocean Master respectively, demonstrating the true fearsomeness of each, and I enjoyed those books considerably. However, I thought it quite refreshing that Johns created a new Aquaman rogue for his final volume, one that I thought had a pretty compelling character and visage.
Of course, Scavenger Peter Mortimer is not original to this book, but is rather an established Aquaman bad guy dating back to the late 1960s (pre-dating Kesel's Scavenger). What turns out to be Death of a King's "big bad," the forgotten Atlantean king Atlan, is also a re-imagining of a character from the Peter David days. Even so, getting out of "Super Friends" territory raises this book a notch; we know Black Manta will act badly and Aquaman will have tortured brotherly interactions with Ocean Master, but with Scavenger and Atlan, the book is harder to predict.
Whereas Johns has done well already in giving Aquaman something of a supporting cast -- Mera, Dr. Shin, Vulko, and especially the Others -- Death presents what feels most like a fully-realized world. This is in part because Death takes place almost entirely underseas, for the first time in this series' iteration, and so we're immersed in the Atlantean people and culture, as well as Aquaman's dealings with sea creatures both familiar and alien. But also, Johns simply has a lot going on in this story, with separate stories for Aquaman, Mera, Scavenger, and a band of Atlanteans lead by Tula (the late pre-Flashpoint Aquagirl); the pages in which scenes from the various storylines were interspersed were quite exciting and well-done.
In true Johnsian fashion, the bad guys here are not really all that bad (as I've noted in copious of Johns's stories before). The Scavenger is basically irredeemable, but Aquaman learns that the vengeance-crazed Atlan was once a kind and open-minded ruler who was seemingly killed by intolerant factions, who turn out to be Aquaman's ancestors. As such, Aquaman works toward peace between Atlantis and the surface world now, but his ancestors believed in Atlantean supremacy and deposed a king who also wanted peace. Aquaman is in a position to sympathize with the mad king (as he was with Black Manta and Ocean Master, as Green Lantern was with Sinestro, as Flash was with Hunter Zolomon, and so on) even as, of course, he can't allow the destruction of Atlantis.
The book's biggest surprise is of course that moment where Aquaman wakes up and finds out he's been unconscious for six months. I hadn't heard anything about it or expected it, and so the moment carried all of its shock value; this was also a perfect way for Johns to reintroduce "the beard," at least temporarily, without simply positing that Aquaman's razor got rusty. If anything, I might've liked to see Johns take this a little farther; there was not much in terms of a new status quo for the characters "six months later" (Mera had sat in jail the whole time, etc.). Also the timing is a bit funny with Trinity War; maybe some might prefer the stories remain separate, but Johns has pretty consistently lined up the characters adventures with Justice League (Green Lantern's exit, Flash being in Flash Vol. 3: Gorilla Warfare during the "Throne of Atlantis" crossover, and Aquaman being with the Atlanteans during Justice League Vol. 4: The Grid), so I was surprised Aquaman's six-month nap didn't have him sleeping through Trinity War and waking up in the world of Forever Evil.
Johns's Aquaman gained traction initially because it humorously and self-referentially took on the perceived weaknesses in the title character and made him "cool" again. Subsequently, especially in Throne of Atlantis, I have continued to enjoy Aquaman less for that humor and more because of how really freakin' powerful Johns demonstrates Aquaman to be. Of course, the fictional character always had this potential to be more or less powerful based on the whims of the current writer, but Johns has done well demonstrating Aquaman's undertaking considerable feats of strength (I thought Johns did a similarly good job in this area with Superman circa Superman and the Legion of Super-Heroes). Here, especially, we see that in Aquaman telepathically controlling the enormous sea-creature Topo to the point where Aquaman puts himself in a coma; this is also significant because it demonstrates Aquaman's mental prowess, not just his strength.
But Death of a King also works because Johns gives us a new take on old Aquaman tropes. We've seen Aquaman as the reluctant king before and we've also seen Aquaman series set in Atlantis before; Aquaman done as a royal drama is what put me off Peter David's Aquaman series toward the end, I'm sorry to say. In Death, however, I think Johns makes a wise and intentional decision to have everyone really not get along -- Aquaman's troops don't like him, no one in Atlantis likes Mera, even the historically Batman-and-Alfred-eqsue relationship between Aquaman and Vulko is strained, not to mention Aquaman can't even get along with the Sea Devils. This removes from the book a sense that everything will be OK in the end (arguably, a strength of the New 52 initiative), and makes for me the court politics more interesting.
There's been a strong romance component to this book, and Johns does give a nod to that as he concludes in the seeming breakup and then reuniting of Aquaman and Mera; this is cute, though the fact that Mera's origins are still tied up in muddled pre-Flashpoint Brightest Day storylines makes it somewhat hard to understand why the Atlanteans don't like Mera and both why she leaves Aquaman and why she comes back.
Curiously, however Johns hardly treats this as an ending per se, even though it's the end of his run. He introduces the Atlantean Swatt, who can't apparently breath under water but has electrical powers due to some origin Johns alludes to but never fully explains. Similarly Johns introduces the Green Lantern-esque Seven Seas concept, suggesting Aquaman could spend some future adventures seeking out the various lost kingdoms of Atlantis (read: multi-hued Lantern corps). All of this is seemingly left for new series writer Jeff Parker to take or leave as he sees fit, but it's a surprising amount of material (the Seven Seas, mainly) for Johns to introduce and then drop, essentially.
In his use of the Trench at the end of Aquaman Vol. 4: Death of a King Geoff Johns brings his Aquaman run full-circle -- the relics of the Others, the battle in Throne of Atlantis, it all ties in here. Ultimately, I think Aquaman Vol. 1: The Trench is the weakest volume of this overall-strong series, perhaps only because it weighed character a little heavier than plot (Trench wasn't bad at all, it's only that the subsequent volumes each improved on it). But it seems to me that an Aquaman by Geoff Johns Omnibus, collecting this mostly self-contained series plus the Justice League issues, would be quite a good read -- that to read Trench through Death of a King all together would feel like reading a well-produced and well-thought-out story that ties together from beginning to end.