I LOVE Old Yeller! Even though I hate the ending, its one of my favorite dog movies...

Lassie Come Home is a close second as well...

I totally forgot about The Ugly Dachshund until bckrazy mentioned that one.. I remember watching that one at school in 2nd grade for some odd reason..

Other movies I like...

Turner & Hooch
K-9
Far From Home
Balto
Beethoven 1 and 2
The Incredible Journey (the original one - again I remember watching that in elemtery school for some reason)
White Fang
Call Of the Wid
Homeward Bound
I like the first Air Bud movie..

And a non dog movie, but a animal (cat) movie I LOVED and used to watch all the time when I was a kid "The Three Lives of Thomasina".. my first pet was a cat, and I was crazy about this movie... would watch it all the time.