eMac 1.25GHz by far the worst
Cube has been my most reliable
I'd second with this. I've just been given a bad G5 iMac and 6/7capacitors are bulging. There's plenty of them on ebay going for spares/repairs (instead of still working).
I have a G5 liquid cooled in the shop right now because of this problem - So that said my addition for the list of most reliable is the earlier air-cooled G5.
The Mac IIcx appear to have had bad capacitors. I know of someone desperate to find working IIcx to keep his old 'production' printer going. For many years any working IIcx's were snapped up by him and he went through old IIcx's very often.
Of course there's also the Powerbook 5300. It's video cable was frequently failing due to it's very bad overall design. (Not to mention the exploding battery issue!).