Zuiko Prime Lenses

Hi all, I've really enjoyed reading a lot of the questions & more importantly the intelligent & informed answers that have been offered.

So here's my question. I'm about three months into purchasing my om-d, & like a whole lot of you guys, I love it. I've got the kit 12-50mm lens & a 75-300mm lens. I'm looking to buy a prime lens & take advantage of the 20% rebate from Olympus we have here in Australia at present.

So my dilemma is which lens to get, trying to work out which one I would use the most etc. I know that I'll get lots of responses in regards to it  depends on what my focal length preference is.... The point is I don't know yet!

I enjoy Streetscapes, architecture and shooting images of the family as we all do.

So as an allrounder prime lens before getting into purchasing all lenses that are available, should I look at the 12mm, 45mm or the very sexy looking (& feeling) metal 75mm??

Any feedback would be greatly appreciated.

AJW71