I really love the last 5 minutes of a photo shoot.
In the last five minutes of a shoot, especially with kids (not toddlers, but the 4-to-14 set), the kids have relaxed faces, they don't flinch at the shutter sound, and they magically seem to blink less! (Not medically possible, but I swear it's true!)
The last five minutes are always full of surprising candid shots for me. I love to see what can happen. We're comfortable, no longer nervous, and usually well on our way to becoming friends. So, there you have it: my very most favorite part of the shoot? Always, always, the last five minutes. :)