Sephiroth in FFVII. First time I played a FF game, so no fancy stuff like that Knights of the Round, barely strong enough to take him on. Took me half an hour and my entire supply of megalixers and ethers, lot of phoenix downs. T'was glorious.
Finishing him off with omni-slash, which I had never seen before, was just bloody awesome.
Guess you can rank the weapons as the others. They were pretty tough unless you really grinded up to high level.
* ah there's also that materia keeper, just after visiting Cloud's hometown (or was it Zack's?). Was stuck there for ages, untill I realised my fire magic was healing the bugger.
That robot just before taking the sub took a few tries too, did a lot of damage when he fired Lapis Lazuli.