The fastest way to treat a low BG is pure glucose, and not just any "sugar". Glucose tabs or gel, glucose candy (smarties, etc) are therefore the best.
I looked up the glycemic index of dates at glycemicindex.com, and dates were only about 50 on a scale of 100 for glucose. Sugar (sucrose) is about 60 on this scale. So clearly dates are not as fast as pure glucose but similar to table sugar.
But that said, I have found dates to be one of the faster fruits to bring BG up if it isn't so low that it feels like an emergency (when my BG is in the 50's to 70's). I routinely eat fruits of all kinds to balance BG. Dates are convenient since they don't spoil and the sugar is more concentrated than other fruits. I always keep a container of them around. Each of the dates I buy are about 6 g of carb per date, so they're pretty easy to measure. Raisins are similar in glycemic index and also good to bring BG up, but are a bit harder to measure - about a dozen raisins is around 5 g of carb.
If I'm going hiking or exploring in a new city where I'll be walking for a very long but unpredictable amount of time, I have made it a habit to carry raisins or dates with me, in addition to the emergency glucose I carry. Great to munch on if you feel your BG going low because of the unpredictable extra exercise.