Batman: Night of the Monster Men and Batman: Night of the Owls, given that each "night" marked both the first crossover and first Bat-event of their respective eras, Rebirth and the New 52.
A fairer comparison might be with Night of the Monster Men and Trinity War or Robin War, given that Monster Men is an inter-title crossover like those others and Owls was a main story in the Batman series plus a string of other-title tie-ins. But whereas it might seem a benefit that Owls allowed the reader to pick up as many or as few tie-ins as they liked, versus Monster Men requiring the reading of all six parts, the Owls tie-ins felt in some respects shoehorned into the crossover and the total result was lesser, not greater. Owls essentially had the Bat-family tackling one threat simultaneously, but separately; Monster Men sees the already-disparate casts of Batman, Detective Comics, and Nightwing truly joining forces against a common threat.
By some logic Monster Men is the "worse" way to handle a crossover, derailing the main plot of three series for two issues each. But given that Monster Men only takes up one month each of these double-shipped titles, and that the crossover falls neatly between the first and second volumes of most of these series, and that most importantly Batman: Night of the Monster Men is really good and arguably fairly necessary, it marks a fine start to crossovers in the Rebirth era. There's a reason crossovers are so ubiquitous -- they're pretty darn exciting, and must sell well, too -- but they fail when they're bloated and don't accomplish anything; Batman: Night of the Monster Men is a reminder of what a crossover can be.
[Review contains spoilers]
Night of the Monster Men introduces Batman's Duke Thomas and Gotham Girl Claire Clover to Detective Comics's Batwoman, Spoiler, Orphan, and Clayface, and pairs all of them with Nightwing for good measure. Though Bat-family adventures are a dime a dozen and none of these are historically new characters per se, there's an extent to which the Batman crew in Rebirth doesn't look quite the same as it did not so long ago in the New 52's Death of the Family, and it's not the worst idea to put them all on the same page together. Though we're talking about three titles where Batman stars or appears regularly, each book has its own distinct cast, and so a large part of the joy of Monster Men is pairing Batwoman, Nightwing, and Gotham Girl, for instance. The book is a true crossover in that sense of the word.
Though not overtly stated, some of the narrative "why here, why now" of Monster Men can be attributed to the recent apparent death of Red Robin Tim Drake. I'm reminded here too of some of the awkward, maudlin ways that the Bat-titles reflected Damian Wayne's death in the New 52, in large part because it seems the event struck some of the titles by surprise. After the emotional scene at the end of Detective Comics Vol. 1: Rise of the Batmen, Monster Men forgoes tears (and any scenes of the extended Bat-family learning about the incident) for a weirdly but refreshingly subtle wake. No one stops too long to reminisce about Tim, but he's mentioned a couple times and he's clearly in the foreground of Batman's decisions during the monster attack.
The importance of the art in Monster Men can't be overstated. Thinking back again to Night of the Owls, it's not so much that the art is uniform throughout those New 52 books as that it all more or less conforms to comic book house style. Monster Men is a gauntlet thrown, not simply in terms of the quick, in-and-out, unobtrusive crossover, but also the look of it -- daring to be different than any of the individual titles, and also tending toward an indie sketchiness. Constantine: The Hellblazer's Riley Rossmo rules the day, with a light but hip and stylized approach that sees a Batwoman with as absurdly long bat-ears as Batman's are stubby. Andy MacDonald's figures are equally uniquely thin; Roge Antonio's work is maybe the most traditional here, but his big-eyed characters remind at times of Rick Burchett drawing the animated Batman in comic form. All of them draw fantastically grotesque monsters. That DC deviates from the standard style of each of these books so soon after launch is daring, and based on look alone Monster Men is a book I'd keep around to read again just to while away an afternoon.
I have been slow to subsume the hype of Rebirth, in part because line-wide tonal change is often something easy to promise but hard to ensure across every single book; more often we end up with some books that work and some that don't, and that's just the way it is. But if Rebirth offers more camaraderie and friendship, it's no more apparent than in the dynamic relationship between Batman and Batwoman, which shown in Detective and continues here. Even as Batman is still keeping things from his apprentices, his ceding of training his team to Batwoman is about the most mature decision Batman's made in decades, and Batwoman's expert teamwork with Batman and her penchant for telling it to him like it is speaks volumes for her character. That the two now know each other as family is a great development, making Bruce effectively no longer the "last son of Wayne," and their partnership does bespeak a brighter, more cooperative era of the DC Comics characters.
At the outset of the New 52, I also remember being significantly impressed with some of the technology writer Scott Snyder was bringing to the Bat-table: digital face-changers, virtual Bat-cave holograms, and so on. I had that "the future is now" feeling again in Batman: Night of the Monster Men, between Batman's VR cowl camera, Bat-beacon messages coming out of every Gotham streetlamp, a Batman armor made of Clayface, Wayne Tower mecha-cannons, and so on. For me, Monster Men succeeded on every level, from Nightwing solving the mystery with Spoiler to a climax that sees Batman versus Bat-villain (though I would've liked some more Rebirth-continuity background on Hugo Strange). Monster Men is the first instant classic of Rebirth; good on you to all involved.
[Includes original and variant covers; Riley Rossmo monster designs]