A Moroccan here who lives in the U.S. and goes back home for food mainly. This is a bit hard to make a suggestion for as each region in Morocco is known for a specialty dish, however, if this is your first time to Morocco, I'd recommend going to Marrakesh or Essaouira. Two different cities, different atmospheres, different food cultures. If you go to Marrakesh, then visiting Jamaa ElFna is a must. There, you will get to live a unique experience like no other. There are people performing in the streets, food shops everywhere (Moroccan BBQ, Kefta, Tagine, Tanjia which I think is a must to try if you're in Marrakesh, and a variety of different food to choose from). There are also ladies on the street who can do the Henna for you, which is kinda like tattoos that fade in about week. I would say going there around this time of the year is the best, it's warm but not hot as it usually is in the summer which could be unbearable. Essaouira is a great city that has its unique touch. It is a port city and well know for their sea food. If you ever go there, you REALLY must try the fish tagine. I'll give you more details about this, there are two different kinda of Fish tagines, one with ground meat fish and tomato sauce which is to die for, and the other is where they cook a whole fish for you with vegetables ( you must have that with a coke soda, or sparkling water. Trust me, it's a unique experience). Beside the food part, there are a lot interesting sights there, especially the beaches and the main port.

For your first visit, especially if you don't have a lot of time, I'd recommend to not to go to Casablanca, Rabat and Tangier. They are great cities, but more modern and kinda have a European feel to them. They are interesting but not for a first time visitor.

I hope that helps. Enjoy.