So..

Chiyo Aiba x Beel?
Futaba Kanzaki x Beel?

I vote Chiyo all the way.

In a world where troubled young hearts are besieged by evil spirits, only one can wield love as a weapon: the God of Conquests, Keima Katsuragi.

... Chiyo for only one reason:

It's the closest Izou will ever come to starting a family with Aoi (after she and Tatsumi get together).

Really? I'd vote Futaba, not because she treats Beel sort of like a slave, but because Chiyo reminds me of typical clingy shoujo characters.

Also, you can't help but feel she has an extremely unique personality for someone her age and she's just so cute.

HikaruYami:

Hah, hadn't thought about that, but it's true.

Would that make Tatsumi distantly related to Izou somehow (not by blood, but by law if you consider Beel his adopted son)?

CheshireCaine:

Going the Tsundere route, eh? Can't really disagree with Tsundere generally. Even though I disagree with your choice, I respect the Tsundere. 😀

In a world where troubled young hearts are besieged by evil spirits, only one can wield love as a weapon: the God of Conquests, Keima Katsuragi.

I'm going to guess Chiyo, although I don't really care that much; I just want more Aoi.

"People don't change, they just give in."Kubera is the best.同人音楽 (doujin music)

I will go with Futaba, just because their interactions are funny and interesting to see compared to Chiyo and Beelz's relationship.
And that is also the reason I root for Tatsumi and Hilda, even though I love Aoi. Actually, it was quite hard for me to choose between both girls, but in the end, I prefer the lol and the interesting stuff. Aoi and Oga are cute but would be quite boring together, just like Beel and Chiyo.

... Ah, I just noticed that I root for the most complex relationships xD Well, where would be the fun if everything is all pink and silk...?

Enjoy life as if you will die tomorrow, but still in honor and respect of yourself.