What is the best way to ride bike?

faster

Just keep the pedals going around, nice and steady. You could try going off road, gravelly tracks require more effort than smooth tarmac. If you want to work harder without changing your route you can stay in a slightly lower gear and spin the pedals more quickly or spend some time sprinting fast and some time at a more relaxed pace.

Mountain Biking

Mountain Biking

This +1, I burn a ton of calories when I got MTBing (for example yesterday 1403 calories in 133mins), and on the top of that if you go out with some mates you will have a laugh)
JC

Higher gear if you have gears. I tend to keep mine on the big ring, even when uphill (if possible).

Be careful with this. Grinding in a big gear will make your muscles work harder and get stronger but if you don't have a good technique or a well fitting bike you could hurt your knees. If your focus is on working your heart and lungs, I'd stick to keeping your pedal strokes smooth and quick.

Edit to add - Remember to keep your pedals moving. You can even pedal going downhill, not to power your bike but just to keep your legs moving. If you coast a lot, you'll probably be resting more than you know and your heart rate will plummet.