We are thinking of going to Morocco in May. Which cities would be most interesting and why? Thank you!
Lots of people skip Rabat, but I'd argue that it's a great place to start. It's much calmer than some of the other major destinations, which makes it a good way to ease yourself into the whirlwind environment that can be Moroccan travel! You can see cool ruins called Chellah, the Kasbah of the Oudayas, Hassan Tower, the old medina, and more. You'll get the best prices for souvenirs here as well.
Train travel is super easy from Rabat, especially the updated Rabat Agdal train station (pro tip - their escalators are basically always working - Rabat Ville, not so much). I'm partial to Fez over Marrakech because it's less intense for foreign tourists and often has a "realer" feel than Marrakech. Marrakech is gorgeous but in my opinion hard to travel in because there's so much hassling that goes on in the tourism industry. Chefchaouen (the blue city) is gorgeous and worth spending the night, especially because then you'll get time there without the day trip crowd. It's harder to get to as there is no train service, so bear that in mind!
Some folks feel really strongly about going on a desert tour on camels - I've done it twice and I think I'm good now. It will be breathtaking while you're actually in the desert, but depending on your starting point you'll spend the vast majority of your time in a bus. May or may not be worth it.
Other places I visited on my trip 25 or so years ago were Meknes, Taroudant and Ouarzazate. I took a public bus over the Tizi 'n' Test pass. Essaouria is a coastal town I haven't seen that others sometimes mention. I haven't even looked at a Moroccan guidebook since my trip, so there may be other places on informed travelers' radar these days, but I was happy with my choices.
Skip Casablanca unless you need it for logistical reasons.
I took a 2-week tour, and compared itineraries with other tour companies. First of all, none go to Tangier. All go to Fès and Marrakech.
The only place we visited in Casablanca was the Hassan II Mosque, remarkable mainly for its size. If we hadn't flown into Casablanca, we would have skipped it. We spent less than a day in Rabat and saw all the highlights. There are more than a few towns and cities that are well worth seeing, but they may not be easy to access from one base. My tour spent 4 nights in Marrakech with only one day in the city. The day trips were all with a lot of travel time. But the best of Morocco is not the cities, but the mountains, the desert, the seashore and the villages in them.
Essouaria is worth a visit - a bit touristy, but lots of atmosphere - a small, charming medina and an active fishing industry. I agree with Chani - the mountains (anti-Atlas, middle Atlas and high Atlas), gorges (Todra and Dades) and small towns like Taroudant, Tafraout and Erfoud are most worthwhile. We took a 2 1/2 week tour in April this year, that wound its way throughout the mountains and included Rabat, Casablanca (for the mosque), Fez and Marrakech as well as all the smaller towns. We had the obligatory camel ride into the desert and overnight at a bedouin camp, but it was very touristy and while the desert experience was ok, I personally do not need to ride another camel. Ever.