If legendaries count; Eye of Sulfuras. Had that item drop a few times in the past (in only about 20 runs at the time as well), but deleted it as I thought I wouldn't need them as I had one on my hunter for the achievement already anyway... ...Later on I realized I can't get the achievement for the item on my hunter as he can't wield maces, which you have to be able to do in order to gain the achievement
I've been farming Ragnaros weekly for months again ever since on 5 characters, but it just won't drop again