Just in case this person is really in a hurry every time (1), I would not give any reason/excuse, because that may backfire with any possible fake / true counter-argument.
I would just let them know that "my time is as valuable as theirs" (in spirit, not with words, as this is most probably seen as aggressive).
It means that I don't see any reason why I would let them be/feel more important than me, or why I would be the one being nice, or even be the one that makes the effort. We both have reasons, personal ones. As sad as it can be: 1st on line, 1st to go.
Sorry, but I am in a hurry too.
Not more, not less. Door's closed. No need to explain, no need to argue.
You say no, set boundaries, and avoid conflict most of the time by being straight to the point.
Basically, @Hanky-Panky's 1st paragraph (2) points out a very good point, and I agree. I would just avoid the (white) lie, and/or give any reason.
1. A friend of mine was switching jobs (2 different companies) every evening: finish at 06:00 PM, start at 06:30 PM (30mn to stop, grab a sandwich and a bottle of water form the small grocery store, eat it and back to work. Always in a hurry, always looking for the smallest line. Sometimes, had to let the food and move without buying because of the time running. She would not clock in late, so she'd rather not eat.
2. (from Hanky-Panky's answer) Since the person who asked you for a place used urgency as an excuse, he/she would not be able to argue if the same reason is presented to them in return. And by keeping it short and to the point, you take away any chance of a counter argument.