The smallest lens for the OM system is probably the pancake 40 f2. It's pretty rare though.
There's a whole bunch of wide angles a bit smaller or the same as the 50 f1.8. There's the 21 f3.5, the 24 f2.8, the 28 f3.5 and f2.8, and the 35 f2.8. I think the 18mm is pretty small too, but rare and expensive. I guess there's also the 16mm fish eye, if you're into that sort of thing.
The big deal lenses are generally the 21mm f2, and the 85mm f2, the 21 simply because it's an f2 but still remarkably small. The 85mm is really small for being an 85. The 50 f1.4 with a serial number above 1.1 million is considered really good. The 50 f2 macro is nifty since it's fast enough for normal shooting but can still go macro.
In general, most focal lengths Olympus produced came in an f2 and they are very very good. Personally, i've got the 24 f2.8, 35 f2, 50 f1.4, 50 f3.5 macro, 85 f2, 135 f2.8 and 300 f4.5. I find i might as well have the 35 glued on. I use the others a lot, especially the 24, 50 and 85. If i was going to get one other lens, it'd be the 21 f2, i just can't find one in my price range.
To me, it's not the lenses or the bodies, it's how the whole system gels together into an incredibly smooth fast way to take pictures. I've never got that feeling from other cameras.