Depends on the type of plane you will be flying

Ultralight F3A planes....2cell 400-800mah
Standard foamies....3cell 800-1320mah
Small balsa planes, and some foamies...3cell 2100-2500mah

I think the most common lipo for most electric pilots is the 3cell 2100mah lipo...we all have em, and we all use them. And most of us use them in multiple different airplanes.