The greater Old Faithful geyser system consists of the Lower, Mid and Upper basins (the lower basin being the northernmost - go figure). They are all popular, with Old Faithful being especially popular. After seeing the sea of completely packed parking spaces in the Old Faithful parking lot, I decided to move on and do come back on another trip.
Lower Geyser Basin
The main geyser in lower geyser basin is Great Fountain Geyser. This is accessible off Firehole Lake Drive, a one-way road headed north.