What is the best way to see a list of leads who opened an email?

You can do this a couple of different ways.

If you are simply looking to see the people that opened an email I would suggest creating a Smart List and adding a filter for Opened Email, within that filter add the email name. One you have that set you can click the People tab of the Smart List.

The second way to do this, and I recommend this way if you plan to run some sort of flowsteps for these people, would be to create a smart campaign. In the smart campaign you want to add the same filter, Opened Email, and add the email name. Under the Flows tab you can add the action or flow step you want to happen and then click the schedule tab. On the schedule tab you can see the number of people that qualify for this smart campaign, which is the number of people that opened your email.