Homs
Homs has been a city of emphasis for the rebels since 2011, when they decided they would try to overthrow the president of Syria, Bashar al-Assad. At first, it looked like the rebels would claim the city, but by May 2014, the Syrian government had taken back the city.
Damascus
Damascus is the capital of Syria, and would be a pivotal victory for the rebellion if they took over. Currently, it is not in control of either side, but it could change the course of the war if this battle was won.
Aleppo
Aleppo is Syria's largest city, over half of it has been destroyed from the warfare. There has been fighting here since July, 2012, and if it is claimed by either side, it would be a major turning point in the war.