For me it was the M60 machineguns from the Battalion armouries. For almost two decades they'd had almost no maintenance from a competant armourer. Instead of replacing worn out parts with new, our armourer had the brilliant habit of storing said used parts for reuse in other weapons at a later date.
1991 rolls around and we're at Canungra. Out of the 9 guns we took with us, all 9 were found to be completely unservicable and dangerous by the JWC (Jungle Warfare Centre) staff. Two days later were were handed the guns back and to our great amazement, no more UD's (Unauthorised Discharges), parts falling off, jams, etc. They were still bitches, but at least they were bitches that worked.
Unfortunately, the other 30 odd M60s left behind in the battalion didn't get the same loving attention.
A couple of years later and we were issued with M16A1's which had come from a unit recently converted to the Steyr AUG. These were to replace the 9mm F1 SMGs and M79 grenade launchers (with M203s).
After the M60s, I'd have to say these were the worst weapons
EVER! Not a single one was anywhere near accurate, most having barrels which appeared to have been used as prybars, sights knocked completely out of alignment and most scary of all, you could grasp butt and stock and twist the weapon - the whole thing would shake and rattle with every shot.
Basically they made the M16EZ of 1st ed look like high quality engineering...