I always thought "Trainspotting" but I definitely liked it more when I was younger. I just don't care about any of the characters (no, not even the dead baby). I wasn't exactly knocked out by "Shallow Grave" but it's at least as good as "Trainspotting" IMO so it's worth checking out, especially if you like dark humor. "A Life Less Ordinary" was shown to me by an ex years ago and it basically just seemed like a chick flick. A *stylish* chick flick, yes, but a chick flick nonetheless.
The only other film of his I've seen is "28 Days Later" and I agree that it's his best and like it more that you seem to- I own it and consider it a unique and effectively creepy take on the zombie apocalypse scenario and one of the best horror films of the last couple of decades.