The prime vs zoom conundrum.
Micro-Four-Thirds is about being compact, so why do we see so many Panasonic/Olympus zoom offerings ? One has a choice of 17 Panasonics and 12 from Olympus. Agreed they are versatile and the more expensive ones are faster and sharper. BUT, with a few exceptions, they are lumpy and, to my mind, defeat the object of having a compact, mirrorless camera body.
As for the primes, three are 25's (nifty-fiftys), one a short-tele at 42.5 (85mm equivalent) and two are 45's (90mm equivalent). This leaves a 12 (expensive), a 14, a 17 and a couple of 20's if one excludes the "fish-eyes". Not a great selection for someone looking for a handy landscape/architectural prime. But the newly arrived the Pana-Leica 15 may be the answer. Hopefuly, it'll be as crisp as the 20/1.7 which I love and it is only a tad larger.
I had enough of the 50's (the nifty one's) in my film years and eventually migrated to a 35mm w/a (±17 in MFT language). I found the fifties too tight for interior shots and of little use outdoors, all they had - back then - was a bit of speed. But that's me and I am aware that we ALL have differing photographic interests.
As OzRay has suggested, with modern digital cameras, and clever software, one can use short telephoto lenses (roundabout 100mm equivalent) to great effect in landscape photography. I have had a lot of fun with my SMC-Pentax 50mm/f1.7 mounted on my Panasonic G1 - it's a very handy landscape photo combo.
But, getting to the point (at last), to answer your question: "If you had to pick one 4/3 prime for landscape, HDR and some architectural photography what would you pick? ". I'd wait for the Panasonic Leica Summilux DG 15mm/1.7 ASPH (to quote it's full pedigree).
It may be what you are looking for.
Dave