My first move out was Mitzi... I let her go because I wanted to let all the animals go who asked to move. I've changed my mind on that haha. I wish to get her again once I've cycled trough the 16 villagers!
Rocket was the first to leave and she was also one of the first to move into my town. She was really nice so I wasn't so sure about letting her go at the time.
Vladimir was the first to leave my town, I was happy to see him go since he was so rude! He's living in one of my friend's towns now, we have no idea how it happened since she hadn't even spoken to him last time she visited...
Skye was my first villager to leave. When I started I had 3 wolf villagers, and even though I liked them all, I said whichever was the first one to want to go, I'd let them go. It was Skye, and at the time it was sad, but seemed like a good idea. But now I regret it. She's always hanging out in my shops though so I get to see her.
I've never let any of my villagers leave, however much I dislike Penelope. I'd feel bad about it, plus I don't want to risk a villager moving in and destroying my hybrids >.>