I already tried almost all lenses of the OM system, except the exotic long tele-photos.
The wide-angle ones are as good as they get, especially the 21mm f/2.0, 21mm f/3.5 and 24mm f/2.0. The 21mm f/2.0 is just surpassed by the Contax Distagon T* 21mm f/2.8.
What's the problem with the 21mm f/2.0? Well, it's very, very expensive.
The 24mm f/2.0 is wonderful and, if you don't need something wider, you'll love it. I just sold mine to buy a Distagon T* 28mm f/2.0, which is rarer and produces slightly more appealing images to my eyes, but it's not that better and the Zuiko is a lot cheaper.
The 28mm f/2.0 is also a gem, but I find the 24mm version to be better, at least the copies I tried. The 28mm f/3.5, for example, is so cheap and incredibly sharp! If you don't want to spend too much, go find one and you won't regret for sure. Get the MC version (no chromed front ring).
The 35mm ones are nice, but I don't find them as amazing as the others. The f/2.0 isn't sharper than the f/2.8 version and it presents a lot of chromatic aberrations wide open.
At the time I had it, I also had a Leica Summicron-R 35mm E55, which was way better!! I know, it's not a fair comparation.
The 50mm models are good, especially the f/1.4 ones. The f/1.8 is ok, but not amazing in my opinion. I tried probably 7 or 8 copies and none of them really pleased me. The better version is the second, but don't ask me why. I saw that information in another forum and I confirmed the same myself.
The f/1.4 worths the difference in price. My brother uses one (multicoated) regularly in a Canon body and the results are really nice. I love the color and the "bokeh". It's not as sharp as my Planar T* 50mm f/1.4, but the price is also in a different league.
The f/1.2 models (50mm and 55mm) are very cool, but they aren't as sharp as the f/1.4 ones, in my opinion.
About the longer lenses, at the moment I have a 135mm f/3.5 and I still can't believe how sharp it is! I check 100% crops from my 5D Mark II and the detail is incredible for such a cheap lens. I used to have a Planar T* 85mm f/1.4, but I rarely pass the 50mm barrel except for sports, so this is a very cheap way to have have a very good portrait lens. Probably I was lucky with this copy.