I don't know if this really counts but it was the first thing that sprang to mind...
I love the scene in Layer Cake leading up to Jimmy's death. The music is perfectly chosen to mirror the tension going on in the lead character's mind, a big part of which is down to an offbeat, repeating beep. I think there's a distinct lack of actual sound effects too, hard to remember though, it's been ages since I've seen it.
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Big +1 on "Children of Men" The sequence is towards the end of the film and it is also worth noting that the whole sequence is an extended tracking shot, no edits whatsoever. Superdramatic!
I absolutely LOVE the sequence in "Children Of Men" when the hero is forced to approach and then enter a building at which there is a fire-fight going on. He has no weapon, and his sense of vulnerability is palpable. The 'music' is more sound design than score. There is almost no dialog. Its gorgeous.
If I remember correctly, something like the last 18 minutes of Alien has no dialogue, just ambiances, sfx and a bit of music. It's just Sigourney Weaver running around the spaceship trying to avoid the alien. The film in general is a fantastic example of how careful sound design can evoke strong emotional feelings (in this case, feelings such as fear and isolation). But the last 18 mins or so particularly relies on the soundtrack to dictate the feelings of the audience.
You may also want to look at animations. Wall-E is one that immediately jumps to mind. Again, no dialogue.
I'm also a huge fan of Aviv Aldema's work on Waltz With Bashir.
I love the fight segments in "Shoot em Up". Dramatic, energetic, full without being a wall of sound. Tarkovsky's "Stalker", Foreman's "Amadeus", Wing's of Desire. These are just add-on's to what other people have listed which are also incredible examples of sound design.
THX1138 is a long sweet segment of pure sound design. Hard to describe what's so good about it.. but it's totally immersive. The funny thing is that I hardly remember any images or footage, just barely, but the sound just sticks.
It's George Lucas's first movie and Walter Murch did the sound design :) (or sound montages as it's stated in the title sequence, which by the way is moving in the wrong direction to get you to feel as if you're going underground... oooh)
In Children of Men, did anyone notice that in that last scene that is all one shot a speck of blood gets on the lense and remains on it for the scene. Apparantly that scene took months to set up and they didn't have a budget to reshoot it or remove it in post. Check that scene out.