I haven't bought a lot of lenses but my first two purchases where the 70-300L and the 15-85. These are the two lenses I brought for use on last years vacation. For me the 15-85 is the perfect walk around lens. Minimizing the need to change lenses. Walking around the city - 15-85; hunting for wildlife on urban nature trails / parks - 70-300L
I loved the 15-85mm lens when I had APS-C bodies. After switching over to FF none of the 24-105mm lenses I owned could touch it for image quality. It's too bad Canon didn't designate an L series for EF-S lenses, it deserves the credit.
The 70-300mm is a sweet one too, I was a fan of the compact beer barrel shape. I have the 100-400mm II now but still miss it.