Answers might have to be split into two categories, i.e. the prize you were most proud of, and the prize that was worth the most money.
Okay, for me, the prize that I was most proud of, was probably the medal I got, for winning the running race, at my first ever school sports day (I also won the race the following year, as well). I don't know whether anyone else on here was ever known as the fastest kid in their year, when they were at school, but I can tell you, when you're 5 or 6, the title gives one major player status, out on the playground, at lunchtime. Unfortunately, my reign only lasted for those first two years, as I never won another sprint afterwards.
As for the prize that was worth the most money, that came along around Christmas, 2006, when I won a giant flat-screen T.V., a DVD player, a pile of DVDs, and a painting on a canvas, from one of the big national Newspapers. I won't say which one it was, because, well, there was a mix-up, caused by the girl in the paper's head-office, who was arranging the delivery of the prizes, leaving her job, and being replaced by another girl, who sent out another batch of prizes, without knowing we'd already received one! This meant I ended up getting two T.V.s, two DVD players, two lots of DVDs, and two paintings! It was indeed a great Xmas that year!