I'll get us started with the best video game dog ever—er, I mean, my personal favorite:
Game: Fallout series
Why the dog is great: The Fallout series is one of the most iconic post-apocalyptic settings in video games. I believe Dogmeat played a big part in shaping this fiction from the very outset. In the early games, he was pretty much the perfect sidekick: loyal, obedient, and not quite as prone to blocking your passage through a doorway as many of the other party members you could pick up along the way to defeating the super mutants were.
Dogmeat's role in Fallout goes deeper than something purely functional, however. His staid presence reinforced the feeling of isolation that pervaded the entire world. Fallout has always been a desolate, moody universe. But the first two games were particularly barren simply because very few characters had any spoken dialogue. Dogmeat was the most silent of all—which makes sense, because he's a dog. He wasn't there to question your decisions, to actively support you or offer a dissenting opinion. He was just there. By following you so dutifully (until he accidentally brushed up against one electric force field or another, which he always seemed to do), he reflected the bizarre nihilism of Fallout's world back to player. Plus, he's pretty darn cute in Fallout 3.
Dogmeat is great and all, but he's just one in a long line of amazing characters. There are plenty of other dogs of all shapes and sizes. I mean: as much as I love the Dogmeat, I still haven't seen him single-handedly (single-pawedly?) take out a helicopter like Riley did in Call of Duty. Tell us about your favorite video game dog in the comments below, and please include an image or video of the canine in question.
Have fun, and a happy National Dog Day to all!