The best ones are those that keep the magic alive.
My Grandma used to take us every year, sometimes twice. She bought me a minnie mouse watch with a bright red leather band at the emporium on main street. She bought this when I was in 3rd grade (1983). She told me it was time to wear my mickey for all to see (of course I picked a minnie).
I wore it with pride until I had to have a couple of swatches (remember those, or am I really dating myself) in middle school. By high school the little leather band didn't fit anymore and the watch found it's way into a desk drawer. I pulled it out when I was visiting home one summer from college. Believe it or not, it was still working!!! Well worth the money. . .I brought it to a watch repair shop and had it cleaned, and changed the wrist band. I then wore it every day after that. My Grandma died 3 years later from cancer, but not before we all took one last WDW trip. You should have seen her face though when I pulled this out originally and she said, "you should always wear your minnie for all to see!"
It finally stopped working about 5 years ago.

It was covered in scratches (front and back) and was retired to my drawer, where I can pull it out often to remember. It has been replaced with a Dopey watch.

When my 4th graders at school ask why I wear a dopey watch, I tell them that I will always wear my mickey on my arm for all to see. While they don't understand, I'll never forget.
Do souvenirs get better than that?