The most calories are burned at your maximum heart rate, period. The faster your heart rate, the more calories you burn. There is no question.
Obviously, however, you can't workout long at max... Many people can workout for an hour or more at 80-90% of their max though... the key is to find an effort you can sustain, and then sustain it.
BTW your maximum heart rate is, simply put, the maximum beats-per-minute your heart CAN beat. It cannot be calculated using the 220 minus your age method accurately, nor any other "formula". I'm nearly 50 years old with a maximum of 206bpm... In my 20's it was over 220... So we're all different. You can only determine your maximum by pushing yourself to it.