In reply to glenn0010:
I'm in the same boat as you, having stuck with glass specific for my Panasonic camera.
I want to buy an Olympus 300mm prime but I'm worried how it will interact with my Panasonic camera.
With Panasonic cameras and glass, there is Image Stabilisation in both the camera and glass. However with Olympus the Image Stabilisation is mainly in their cameras. Only the high end glass has Image Stabilisation and sadly from what I've read the glass Image Stabilisation is not compatible with Panasonic cameras.
This isn't a problem, I'm guessing here, with Olympus cameras because the Image Stabilisation is in the cameras and they don't get much glass with Stabilisation to begin with.
As for a One-size-fits-all, I think you are looking for Unicorn Poo, which you'll never find. From what I've read, and again with no facts to back this up, Olympus make better glass. But this is only based on reviews and no first hand experience.
To counter the problem of continuously swapping lenses, have you thought about using two camera bodies with lenses to cover most situations?
One body with a lens for landscapes, and another with a Tele lens on it?
They'd most likely come to roughly the same weight as a reputable Full Frame camera and equivalent zoom lens.