If you want my opinion on it, i'm pretty certain who the Hakurei God really is will never be Revealed officially.
I think ZUN doesn't really sees this as important, it's just a point that you are supposed to wonder about and fill in yourself
In order to make good lore it's important that there are some things that are kept mysterious... and to let everyone have and keep their own idea on the subject.
I think ZUN sees the Hakurei God's Identity as one of them.
Same thing with the relation between Maribel and Yukari, for example...
now if you want my own, personnal take on this:
-My favourite pet theory is that the Hakurei God is no other than.... Reimu Herself! And she doesn't know it...
i've read somewhere (think it was SoPM or WaHH) Kanako said about her she is already God-like, being independent from the rest of the world and residing all alone in her shrine following just her intuitions, like this. And she also said that "if the Hakurei shrine had more followers, she'd have already become a Kami."
Very suspicious! now this could be interpretated as the Hakurei's God powers being sort of "Washed Off" onto her, but Reimu becoming more Godlike and more powerful whould be very well explained if she actually was the god of the shrine: gods become the more strong and wise and important as they gain followers. And then, because the shrine is destitute and has no followers, its god (Reimu) remains greatly weakened (she remains mostly Human-like and can't realise who she is).
My opinion is that she'd be the God of Dream (she use lots of Dream sign attacks, and Dream is linked to Fantasy) or God of Fantasy (= God of Gensokyo: the land for fantasy, outside of the real world). Japanese Gods (Kami) are supposed to be the essence of things like mountains, but also Concepts, ides or emotions, that took a physical form. Reimu is the God that embodies the essence of Gensokyo: The Fantasy/Dream. And her Shrine powers the Hakurei Barrier, which sets the limit between Fantasy and Reality...
and before you say "That's stupid, a Miko (by definition: a Human who serve gods and shrines) can't be a God at the same time!" well, i answer you, Sanae pretty much proves the opposite! she is a god, too: a descendant of Suwako. That, and you also need to read Silent Sinner in Blue! :derp: ( Izunome)
only my 2 cents of course.... :3
Other ideas that i like include, in order of preference:
-All of Gensokyo itself and everyone within it; in other worlds, the Shrine dedicated Fantasy/Mystery in general, pretty much ties in with my Reimu main idea, except that here, gensokyo itself is the god, without having took physical form (cause the shrine is too destitute). Reimu herself, then, is just the Miko serving the god of Gensokyo, who can't manifest. She isn't that god without knowing it.
-Yukari
-ZUN
-YOU! the player (Nice one, Mikuru, i never tought about it, i like this idea too!)