The questions that appear most often in Google interviews:
Design Google Search. Focus on crawling, indexing, ranking, and serving. How do you handle billion searches per day?
Design YouTube. Focus on video upload, transcoding, storage, CDN, and recommendation. How do you serve billion hours of video daily?
Design Google Docs. Focus on real-time collaboration, conflict resolution (OT or CRDTs), and offline support.
Other common topics: Google Maps, Gmail, Google Drive, a URL shortener.