What would be an ideal atmosphere for humans? I have been searching to find out what we can breathe and cannot breathe. The percentages and mixes are lost on me. Could there be an atmosphere more beneficial to us than Earth?
In general, probably not. We have millions of years of evolution based on the percentage we have now.
There are better mixtures in certain situations though. Many spacecraft have atmospheres of very high percentages of oxygen, sometimes even 100%, at low pressures. Oxygen is toxic at high concentrations and normal air pressure, but at low pressure it's not toxic and the low pressure means the spacecraft doesn't need to be as strong.
Divers often use different mixtures to avoid pressure sickness.
In supplementing another answer, another consideration that I can think of is altitude. High concentrations of oxygen can be toxic at higher pressures, so a higher oxygen level for our planet may make things easier for people at higher altitudes, and vice versa. So the overall average altitude of where people live could influence the "ideal". So if you're say talking about engineering a planet or something in a story to be this way, you may want to take this into account, as its "easy" to control the global air mix, but you will run into trouble trying to have local areas have different pressures/mixes in an open air environment. Obviously, this is pure speculation on my part. Additionally, the environment has changed dramatically over the Earth's life, and evolution tends to lag behind it. So its plausible that we are in a 'change' period, and our own evolution hasn't caught up to the new norm, granted, the new norm isn't drastic enough to be killing people left and right, but it is by definition, not 'ideal'.