Dubai
The best in it is everything; people, attractions, rules, roads, parks, cleanliness, ideas...
Any place you go you will be satisfied and happy.
All places are worth of visiting.
Sharjah
The best of it is view of Dubai at horizon
Joking! The best of Sharjah is the rain.
Ok, ok. Let me be serious; the best of it is the forest around golf fields and Museums.
R.A.K.
Mountains are the best there.
Ajman
Al Zorah Natural reserve definitely the best of Ajman.
Fujeirah
The very first mosque in UAE, valleys, forts, the beach.
Abu Dhabi
Qasr al Watan beats it all. Deserts and resorts around Abu Dhabi and White Mosque then Car Museum follows. Maybe add Mangroves to top the list.
Al Ain
Nature, Wadi Adventure, water, plantations...
Umm al Quwain
The red desert and Dream Land. I don't know nothing else there for now.
Liwa
Moreeb Dunes and Empty Quarters, palm plantations.