I've done a fair amount of selling to buy when it comes to glass over the past 19 years but the one lens that has stayed in my kit is my first 'L' lens, the 70-200 f/4L. There's nothing wrong with it, it is sharp, great contrast, light and dependable. In fact if I don't need too wide, that and the pancake are a great lightweight two lens combo.
How about anyone else? What oldies but goodies do you use?