The best cadence was discussed here before. What I personally found is that it depends on what is your weakest part. For me it is the heart (and lungs). Spinning exercises heart more than legs, so I discovered that I ride faster and with less efforts when I mash a bit more than spin. Recommendation that 90 rpms is the best just does not work for me. I prefer around 70 on flats, 60 or less climbing and 80-90 when barrelling down the hill.