My favourite is the OM-4T. I like the Av autoexposure (much the same as the OM-2 and the "consumer" OMs) the spot-meter (the same, I think, as the OM-2SP and the OM-3s) and the multi-spot metering (the OM-4s' specialty). The latter is most useful for landscape or other slow-paced work, though. I think the "T" version has (or is supposed to have) better build quality and some of the electronic gremlins worked out. I've had good use from mine (though not much use for some years now). Oh, I also like the 1/2000th shutter speed (not sure which other OMs have that, but I know my OM-20 doesn't). I had an OM-40, which I rather liked but guess that its early form of evaluative metering wouldn't cut it with modern electronic marvels. At the time, I decided to upgrade to the OM-4 as multi-spot metering suited my (then) uses better.
...Mike