Finally there's list now of the most viewed arab songs ever on youtube.

Not that they are always the best songs, but its an interesting reference. (I got them from the top 50 arab : http://www.statoz.com/arabic )

Top 10:
1 Nancy Ajram - Fi Hagat 20416433 views
2 Nancy Ajram - Mashy Haddy 15520409
3 Cheb khaled - Aicha 11382955
4 Nancy Ajram - Ya Habibi Yalla 11256561
5 Carole Samaha - Khallik Behalak 9369570
6 2pac & Elissa - Ahla Donia (Arabic Remix) 8657194
7 Amr Diab - Tamally Maak 8630778
8 Tamer Ashour - Iftahrikna 8492119
9 K'Naan ft. Nancy Ajram - Waving Flag 8126021
10 Nancy Ajram - Ebn El Geran / نانسى عجرم - إبن الجيران 8019607