CyanogenMod has been the most widely-used custom ROM for years, with at least 10 million users at one point. After Cyanogen Inc kicked the bucket, the community-maintained LineageOS continued CM’s efforts. The official statistics page for the project now shows 515,000 active installs (at the time of writing), a great achievement for the relatively-new ROM.
The statistics page has some other interesting information. The most popular device, in terms of active installations, is the OnePlus One (bacon) with 40,000 installs.
LineageOS passes 500,000 active installs worldwide, OnePlus One is the most popular device was written by the awesome team at Android Police.