I get why people are upset, for the most part. The Chihiro ending(though technically not an ending yet, and has a very small chance to get flipped around next week) was most plausible though, and I don't see eye-to-eye with other shippers. I'll leave out any conquests with no further connection to Keima, including goddess hosts.
I saw Tsukiyo ships, Nikaido ships, hell I saw a Lune ship and an Akari/Rimyuel ship. It should be obvious how these were improbable.
Ayumi: The main argument I see supporting her ship is that she is the first girl, so she must be the last. That's a logical fallacy. But does it fit the theme of the series? Just how well do they click? It's obvious that everyone Keima courted fell in love with him, so she doesn't really have merit there. She was clearly the most upset in the Jupiter Arc, and she even walked away from the circle. She doesn't really have any greater relationship with Keima aside from the proposal which was staged to get her goddess out.
Kanon: Past the Goddess Arc, Kanon has no shot. In it, she was stabbed, so she definitely played a part in developing Keima as a character that must accept his connection to reality and its people, but as far as their relationship goes, she was just a princess waiting to be saved.
Tenri: She definitely had the best chances after Chihiro, considered the last (two?) Arcs revolved around her. But, it's a one-sided relationship. Keima doesn't react to her presence any more differently than with the other goddess hosts. At most, he thinks of her as a friend.
In fact, almost the entirety of the Future Arc are one-sided relationships while Keima's maturing more as a real person. The goddesses are taking care of Keima, but it's the younger version of him in there. Keima's too worried about connecting the dots to reciprocate any real love.
Elsie: This chapter and the last confirmed it; she loved Keima as a brother and nothing more. She's certainly had her strong moments, but ultimately she never showed any signs of the romantic kind of love, nor him to her. Nothing more needs to be said, I believe.
Haqua: Again, past the Goddess Arc, she has no role. Now she's worried about Hell and how she and Nora plan on restoring it. I certainly don't see any lingering thoughts of Keima (but who knows, maybe that's part of the memory wipe).
The Chihiro end makes sense thematically to me. She and Keima are foils of each other; Keima the gamer and Chihiro the most real girl in the series. If the theme is accepting real life, by choosing Chihiro, his is quite literally accepting the real world as his partner, no asking for it to be. Then there's the fact that she loved him or at least had an interest in him before her actual conquest, how real Keima himself was DURING her first conquest, and then the whole ending of the Goddess Arc, where they are the most open they ever have been with each other. Keima cried for what I remember as the first time in the whole manga, then there's that whole line, "I'm sorry Chihiro, that isn't what I meant," which is when SS Chihiro really went full sail.
All that aside, yeah the plot holes are glaringly obvious. No background on Hell or the Algemachina or the war, everyone's memory might be wiped, the letter is left unopened, Lune hasn't showed up at all since she was revealed to avoid being arrested and a part of Satyr, the battle with Elsie was glossed over (there is a theory that by having Keima as a partner, the never even happened and Elsie stopped because she found happiness [basically, Dokuro set it up this way so Elsie wouldn't destroy the world]. I don't this all too plausible) and no reason was given for her disappearance from the past, Keichi hasn't appeared at all (though at this point, it's really meaningless if he never does and remains a simple plot device), etc.
The manga was originally supposed to be 27 volumes, and I definitely hoped the author would have stuck with that regardless of editor decisions, but damn. Hopefully there will be some sort of spinoff that fills in the gaps, heh. At the very least, next chapter is apparently longer than normal, which means at least some things will hopefully be briefly explained in a somewhat cohesive way.