Almost every DT fan would say 'Images & Words', but think about it. For most of us, I&W holds nostalgic value. The production sounds good to our ears, with the triggered drums only adding to the charm.
I wouldn't introduce someone to DT with I&W. Yes, it's a classic, but it sounds way too outdated for someone who's unfamiliar with DT. I'd give them Scenes, as it's the best combination of 'old school DT' and the production is pretty good. Or the experimental masterpiece with awesome production and newfound heaviness: 6DIOT.