The Colossi, Shadow of the Colossus
SOTC could well have been designed as the result of a late-night "What's the biggest boss you've ever seen?" discussion - there are no minor enemies, just bosses. And those bosses are moving, breathing levels.
Each has to be climbed to be killed and the fact that this can take up to half-an-hour should give you some indication of the scale we're talking.
Above: So big, you'd need to turn a widescreen telly on its side to actually get a shot like this
If you haven't played it, anything we can mention or show will only spoil the effect; but if you want to feel like a true giant-killer (and, weirdly, it's not as good a feeling as you might think) no other game comes close to doing it like this.