This questions risks inviting a lot of opinion, so some may vote to close this down.
However, to try and keep this to the facts, I refer you firstly to what Apple has to say about their batteries. You should also read this.
In a nutshell, the key indisputable points are:
- Apple lithium-ion batteries use fast charging to reach 80% of their capacity and then switch to slower trickle charging to complete the charge.
- Charge your iPhone whenever you want. Many people believe you need to let them discharge 100% before recharging. This is not true.
- Ambient temperature is one of the biggest factors in battery health/life. iPhones are designed to work at their best when ambient temperatures are between 0° to 35° C (32° to 95° F).
- Store your iPhone where the temperature is between -20º and 45º C (-4º to 113º F). Don’t leave it in your car, because temperatures in parked cars can exceed 45º C (113º F).
Also, the latest research confirms one of the most important determinants of what ruins a battery is time. (You could purchase a brand new iPhone battery and leave it unused in its original packaging for five years and then find it's capacity is nowhere near what it should be when you start using it)