To reach Kilimanjaro’s Uhuru Peak there are various routes to choose from varying anywhere from 5-11 days. The shortest route isn’t always the best in this case and especially so if you aren't used to high altitudes. The longer the route, the more acclimated you will get to the altitude and the higher the success rate.
Here is a comparison of some of the most popular routes:

Most Scenic Route: Lemosho Route
Least Trafficked Route: Rongai Route (also has the lowest success rate)
Highest Success Rate: Lemosho Route
Marangu Route
Marangu is the shortest route and a popular route too. It is the only route to offer accommodations along the way which may be favored if hiking during teh rainy season. This route is good for people who have experience with high altitudes.
Minimum Days: 5
Recommended Day: 6
Scenery: Good
Traffic: High
Rongai Route
This is the only route up the drier, northern side of the mountain. Due to its gradual slope and slow ascent, it’s better for less experienced climbers and it has a high success rate. It eventually merges with the Marangu route. On this route, you can camp alongside the only lake on the mountain and enjoy spectacular views over Kenya's Amboseli plains.
Minimum Days: 6
Recommended Day: 7
Scenery: Very Good
Traffic: Low
Machame Route
This is a popular route and takes usually 7 days. The route approaches the mountain from the West and offers some beautiful panoramic views. Machame is better suited for those with some hiking or backpacking experience.
Minimum Days: 6
Recommended Day: 7
Scenery: Excellent
Traffic: High
Lemosho Route
Lemosho is a newer trail and is considered the most beautiful route offering stunning views. The trail was designed to give hikers more time to acclimatize to teh altitude, giving it an excellent summit success rate.
Minimum Days: 6
Recommended Day: 8
Scenery: Excellent
Traffic: Medium