Best Prime Lens to Buy in the Format

What he said.

On the wide end, the PL15 F1.7 is an excellent lens, period. But it is expensive. For much less money you can go slightly wider, a stop slower, and less sharp with the Panny 14mm F2.5. Less sharp, but with plenty of people who like it. Full disclosure, I went with the more expensive PL15.

Moving to the mid-wides, You have the 17mm F1.8 (see me note about the 2.8 below), sigma 19mm f2.8, sigma 16mm f1.4 (heavy, very high quality), and Panny 20mm f1.7. All are good lenses. The sigma 19mm is dirt cheap and while slow (f2.8) it is sharp when wide open (at 2.8 it is as sharp as anything else at 2.8).

Standards are the 25mm F1.7 and f1.8 from Panny and Olympus. General consensus online is that the Oly is slightly better, but Panny is much cheaper. PL25mm f1.4 is a great lens (I have one), but the new Sigma 30mm F1.4 is sharper. Sigma also makes a 30mm f2.8 that should not be discounted if you like slightly tighter compositions and want to save money.

On the longer end, the 42.5 f1.7 and 45mm f1.8 are excellent (I have the 45mm f1.8 and it is my second most used lens). The Sigma 60mm f2.8 give you reach that your kit lens doesn't, and while f2.8, can still provide a very shallow DoF (headshots with one eye in focus and the other out). All three are winners. The Panny 42.5 is probably the best bet for you as it has DfD (faster focusing on your body) and a more traditional "portrait" fov.

For your $500 you can have all three Sigma 2.8s, or a Panny 25mm f1.7 and a Panny 42.5m f1.7. I'd probably go the latter, as those two focal lengths make up 95% of the shooting I do.

The only lens I'd avoid is the Olympus 17mm F2.8 Pancake. I say this having owned one. It does lovely colour but handles high details terribly. Detail turns to mush. Panny 14 f2.5, Oly 17 f1.8, Sigma 19mm f2.8, or Panny 20mm f1.7 are all better choices.