Best Telephoto Lens (100mm+)

I have a GX1 with 4 lenses so far:

Oly 12-50mm (planning to replace with panny 14-45)

Panny 20mm f/1.7

Oly 45mm f/1.8

Samyang 7.5mm fisheye

As you can see, I'm pretty well covered under 50mm and now I want to look into the 100mm+ range. Since I have a panny body, I'd like to get a pany OIS lens. I know there are several, but could the kind forum members provide some input/experience into which are better? I'd like to stay around $500 but budget is flexible (just not 35-100/2.8 price flexible).

As a side note, I've found the 12-50mm not crisp enough for my taste, which is why I'm looking to replace it with the 14-45. Or supplement. I may keep it for video/macro.

thank you everybody in advance!