This is kind of a loaded question, and it really depends on what you see in a race track that makes it great. I usually like tracks here in the United States like Mid-Ohio and Laguna Seca. I think Mid-Ohio is such a loved course for it being very well-balanced in terms of speed and handling. Laguna Seca just has a very challenging layout- and I am not just talking about the famed Corkscrew. I like tracks like Magny-Cours and Instanbul Park for being great rhythm-based tracks. I love Suzuka for being a truly challenging and rewarding track to race on. Then, you have classic circuits such as Circuit de Spa-Francorchamps and Autodromo Nazionale di Monza offering high speeds and decisive corners. Temporary courses I've always liked include Long Beach, Vancouver, Toronto, Burke Lakefront Airport, Belle Isle, Monte Carlo, Albert Park, Surfers Paradise, and Bathurst. Last, but not least... you have tracks like Circuit de la Sarthe, the Nürburgring Nordschleife, and the Snaefell Mountain Course (Isle of Man TT). Most of you know I loathe any course that uses the Norschleife, but even these courses have their appeal. Also, some of today's race tracks have had past layouts which seem much more respected and loved than where they are at now (for those still around). For example, more people remember the old Hockenheim where you went full speed through a forest-like setting. MUCH different compared to today's Hockenheim, though its high-speed character remains intact.
So what makes all of these courses great? It all depends on layout and simply what level of challenge and intrigue each course brings. It also relates to what you see in a race track that makes it great to race on. Not everyone feels the same way about every course ever raced. That is at least a professional-style answer to what makes tracks great.