Well, steel_fist_14, you're not alone when it comes to milestones in COD Zombies. I've been playing it for so long, I've lost count of how many milestones I've passed. But if you're asking for one special milestone that hits the feels, I got you. Here's mine:

Getting the ray gun on round one without cheating.

You know that feeling when you see the mystery box light up, hit it and your screen goes white and orange. Then, after what feels like forever, you see it materialize in your hands. BOOM! Ray gun, baby! It's the ultimate weapon in the game, and to get it so early in the game is an unbeatable adrenaline rush.

So, steel_fist_14, keep on playing, keep on slaying zombies, and keep on chasing your milestones - no matter how big or small they may be!