There are 7 UNESCO World Heritage Sites in Sicily for their historical, artistic, archaeological and naturalistic importance. Plus Scala dei Turchi has started the process to become a UNESCO World Heritage Sites.
Necropolis of Pantalica
Valle dei Templi
Villa Romana Del Casale
Scala dei Turchi

There are also 2 sites which will probably become UNESCO World Heritage Sites in the near future
Taormina and Isola Bella
Sicily has many archaeological sites. Excluding la Valle dei Templi and Piazza Armerina (Villa Romana del Casale) which are listed above as they are also UNESCO World heritage Sites, the top archaeological sites are:
Solunto
Sicily has been inhabited for thousands of years by many different cultures, this is shown in the amount of History found in every corner of the island and in the many beautiful Palaces, Town squares (Piazza), Towers, City Gates, Fountains and so on. The most beautiful Palaces and Piazzas in Sicily are:
Norman Palace
(Palermo)

Piazza Duomo
(Cefalu)
Piazza Duomo
(Messina)
Palazzo Mirto
(Palermo)

Piazza Garibaldi
(Caltanissetta)
Piazza Ciullo
(Alcamo)
Piazza della Repubblica
(Mazara Del Vallo)
With nearly 1000 km of coast and more than 100 beaches Sicily has a variety of beaches. From sandy beaches to stretched of rocky coast and gulfs. In Sicily you can find beaches with fine golden sand, sand mixed with gravel, pebbles (from small to large) and even beaches of dark volcanic stones.
San Vito lo Capo
Mondello (Palermo)
Fontane Bianche (Siracusa)
Scala Dei Turchi (Realmonte)
Lungomare di Cefalu (Cefalu)

Vendicari (Noto)
Castles
The top castle in Sicily are:
Lombardy Castle
(Enna)
Zisa Castle
(Palermo)
Sperlinga Castle (Sperlinga)

Castello di Venere
(Erice)
Castello Manfredonico
(Mussomeli)
Castelbuono Castle
(Castelbuono)
Chiaramonte Castle (Siculiana)

Churches
Top churches to visit in sicily are:
Palatine Chapel
(Palermo)
Monreale Cathedral
(Monreale)
Martorana
(Palermo)
Cefalù Cathedral
(Cefalù )

Noto Cathedral
(Noto)

Coastal Towers
The top Coastal towers to visit in Sicily are:
Torre Capo Rama
(Terrasini)
Torre Spalmatore
(Ustica)
Torre delle Mandre
(Altavilla Milicia)
Torre Bennistra
(Scopello)

Torre Pozzillo
(Cinisi)
Torre Nubia
(Paceco)
Torre Cabrera
(Pozzallo)

Nature
Sicily has a huge naturalistic heritage, made not only of beaches, but also of mountains, parks and volcanoes. The top nature sites in Sicily excluding the Etna and Madonia National Parks and Rocca di Cenere Geopark mentioned before are:
Vendicari Nature Reserve
Nebrodi National Park

Pantalica Reserve
Cavagrande Del Cassibile Reserve
Alcantara Gorges
Villages
Sicily has 21 villages part of the I Borghi Piu Belli D’Italia, an association of small towns of historical interest. In order to be part of this association a village or town must meet a number of prerequisites. The villages are:
Province of Agrigento: Sambuca Di Sicilia
Province of Caltanissetta: Sutera
Province of Catania: Castiglione di Sicilia and Militello in Val di Catania
Province of Enna: Sperlinga and Troina
Province of Messina: Castelmola, Castroreale, Montalbano Elicona, Novara di Sicilia, San Marco D’Alunzio and Savoca
Province of Palermo: Cefalu, Gangi, Geraci Siculo and Petralia Soprana
Province of Ragusa: Monterosso Almo
Province of Siracusa: Ferla and Palazzolo Acreide
Province of Trapani: Erice and Salemi