The Northern Circuit is the only route to traverse the northern side of the mountain. It is by far the most scenic and has the highest success rate of all the routes on Mt. Kilimanjaro. We highly recommend it if you have the time.
Machame is a favorite route for a number of reasons. Firstly, it has one of the highest success rates - 90% for a 7 day trek. Secondly, it's great for acclimatization, allowing you to naturally "walk high, sleep low" every day.