Definitely Hilda.
Aoi is really annoying and mean (she flushed the demon that gives her power down the toilet etc). Then in the latest arc she is totally pathetic. If it wasn't for Oga jumping in, she would have allowed Beel in Oga's body to have been hurt.
Aoi was cool in the beginning but as the manga went on she became a weak side character.
I personally don't think any romance will be put in here. Considering that all things love and such is Oga's weakness. I personally prefer Hilda, since she's just as whack as Oga. Besides, usually the protags end up with the first girl, though I don't really mind either way. But hey, Moe Hilda x Oga is my fave xD. It's just so funny how Oga reacts to her.
On a side note, why'd you put Alaindelon?! Everyone knows Alaindelon is meant for Furuichi
There is already romance, though not much. Oga clearly cares deeply for Hilda, the way he goes in singlehandedly to save Hilda against the entire Akumano Academy (Lord En's team). Also, all the rivalry between Aoi and Hilda where Oga is concerned.
As for Furuichi - I think him and Lamia are the pairing.