One of the most popular ROMs with very unique features, based on AOSP.
Ranked in these QuestionsQuestion Ranking
Pro CyanogenMod Theme Engine
One of the most powerful theme engines, used widely among custom ROMs with a healthy amount of themes. Lets you customise almost everything on the ROM, including icons, colour schemes, framework layouts and bootanimation
Pro Beta channel with experimental features
Brings features requested by the community for users to test. Relatively stable. An example would be bringing features such as the overhauled recents feature and heads up notifications from the Lollipop developer preview to Kitkat .
Pro Large user base
With a large user base, any issues you encounter has probably been reported online where others can post solutions.