Best: When my mum got me a Sega Megadrive (Genesis for the US folk) 2 for my birthday. I nagged her for months prior and on the morning of my b'day she said she didn't get me one, I was bummed out but then she gave me my present, woo hoo! We never got expensive presents like that so it was such a long shot. A very materialistic best day, but to this day I still remember how excited I was (come on, I was a kid).
Worst: Same as Batutta. My dad died of cancer years ago. It was one of those best/worst days. Watching someone die is not easy and letting them go is even harder.