Greedy by earliest end time: Always pick the activity that ends earliest among those that do not conflict with already-selected activities.
Why does this work? By finishing early, you leave maximum room for future activities. Each greedy choice is locally optimal and never blocks a better global solution.
Here's the core idea for activity selection. Let me show you why it works. End time is the only metric that captures "leaving room." The earlier you finish, the more options you have for what comes next.