Easiest is very hard to define but if we simplify and take number of turns and gear changes per lap (source: forceindiaf1.com):
Turns per lap
Autodromo nazionale di Monza 11
Istanbul Park 14
Circuit Guilles Villeneuve 14
Hungaroring 14
Sakhir International Circuit 15
Sepang International Circuit 15
Autodromo Jose Carlos Pace 15
Gear changes per lap
Autodromo nazionale di Monza 22
Istanbul Park 38
Autodromo Jose Carlos Pace 38
Suzuka Circuit 40
Circuit de Catalunya 42
Hockenheimring 42
Silverstone 44
Circuit de Spa Francorchamps 44
Monza and Istanbul Park are both on top of the list so learning those tracks from scratch could be easier than learning the others but of course driving a Sunday-drive on an “easy” circuit and making a fast lap on one is a different matter.
Note: Valencia has most turns (25) AND gear changes (66) per lap in the entire calendar. Thinking about how many laps it took for me to get it right in F1 2010 I would never say itīs one of the easiest but definately the most fun street track of the calendar. After learning the circuit it is pretty easy to get it right but definately not as easy as the mentioned Monza and Istanbul circuits.