I've used extensively all of the 50mm Zuikos besides the f2 macro and I agree with others that the f1.8 is something of a sleeper.
I tested many, many copies (over twenty) of them side by side and an F.Zuiko came out the winner thanks to its rendering of shadows and corner-to-corner sharpness. A hair less contrasty than later versions, but its rendering mattered more to me (shadows in later versions were mushy by comparison). Note that there is significant sample variation that I couldn't correlate with model versions, so you may have been lucky or unlucky to various degrees with your copy.
The f1.4 I tested some ten copies of, and later copies were indeed significantly sharper and more contrasty than G.Zuiko versions, but I don't give any credence at all to the ">1 mio" or ">1.1mio" serial number mythology that has arisen around these lenses. An 850,000 lens was the best performer in my comparisons. A good f1.4 is indeed a better performer than a good f1.8, but not by a long way. It's better wide open than the f1.8, but the gap is closed by around f5.6. A good f1.8 is better than a good G.Zuiko f1.4 (and an average probably better than an average).
The f1.2 is a very special lens for me, largely because of the glowy halation it gives wide open, combined with decent resolution there. This disappears somewhere between there and f2, at which point it is decently sharp, and it keeps improving until f8 or so, where it is really stunning. There isn't much to separate it from a good f1.4, but my copy (only tested one) is a hair better at resolving fine details than the f1.4 and has better contrast in such regions.
The f3.5 macro is a fantastic little lens, and without doubt the most versatile of the lot, I suspect including the f2 (considering macro, infinity, compactness and light weight), but it isn't up to the standards of a good f1.8, f1.4 or f1.2 at medium to long distances in terms of absolute resolution, although it still does a very good job in these ranges. Absolutely fantastic as a macro lens. Also, for me, it has the best colours of all the 50s: rich and warm. The f1.4 (non-G.Zuiko) and f1.2 are more neutral, and the f1.8 is somewhere in between.
Personally, as a user and not a collector, I'll probably only keep the f1.8 (travel) and f1.2 (everything else) in the long term, since the 90 f2 outdoes the 50 f3.5 for macros, as good as the latter is, and the f1.2 is better than the f1.4 for when weight and bulk don't matter.
If absolute maximum sharpness at longer ranges and low-light performance can cede to having a do-it-all lens, the f3.5 is a clear winner, and if you don't care about macro, or get that elsewhere, the f1.4 is the best jack of all trades. If you already have the f1.8 though, I wouldn't bother with the f1.4---it isn't enough of a step up in any department.
