Since every country has its own icons, let me share how to build a must-see list that actually works. Start with the official tourism board website and the country's UNESCO World Heritage entries; between them you will capture the landmarks the country itself considers defining, from monuments and old towns to natural wonders.
Then sanity-check that list against reality. The most famous sight is not always the most rewarding, and a few icons everywhere are honestly better in photos than in person. Ask locals or long-term expats which two or three places they take their own visiting parents to; that question cuts through marketing better than any top-ten article.
Practically, resist cramming. Pick one anchor landmark per day, book timed tickets in advance for the big names to skip queues, and go at opening time or late afternoon for thinner crowds and better light. Mixing one famous site with one neighbourhood wander per day gives you both the icons and the feel of the place.