I've visited Morocco two times and I can say that it really is a beautiful country, I've had very good experiences there so if the opportunity arises I would definitely visit again! Two other Arab countries which I definitely still want to visit are Egypt and Jordan. Egypt because it's full of historic sites (like the pyramids) and full of culture (like the ancient Egyptian culture). Jordan because I would really like to visit Petra, the Dead Sea and the city of Aqaba. Furthermore I think that Oman should be a beautiful and interesting country as well.
By the way, @Amruth: Nice to see that you're interested in visiting Iran. I'm also interested myself, have been learning the language for one year now and will probably visit Iran next year. However, Iran is not an Arab country ;)