For me it was the long (I think 18, 24 and 30 inch) 3/8 drive extensions. I needed a long one for removing the battery in my Mother's VW, once, bought the set because it was the only option at Crappy Tire and I didn't feel the need to wait for Amazon (this was several years ago, so they didn't have stellar shipping time in my area). Since then they have hung on the side of my box untouched, keeping in mind that I am a weekend warrior, not a pro.
Ratcheting extractor "wrenches" I have used once or twice (It worked so now of course I want the ones that go on an impact just to have them for a pinch), but by far the biggest disappointment in my toolbox is the sets of universal spline wrenches and sockets. Again, they're for those special cases that seem to never come along (like e-torx). Maybe I'm just not taking on any majorly challenging tasks, which is a distinct possibility.