There is two official route which is safe and recommended by the government and Mount Rinjani National Park officer, That is Sembalun Route and Senaru Route.
Basically, both of the routes is through the same path, so the view is mostly same if you do the 3D2N Full Trail from Sembalun and finish in Senaru (vice versa)
For a trekker who has a limited time, but would like to reach the top of Rinjani, they can start the hike from Sembalun as it only needs minimum a 2D1N to catch the top. but this program requires a good level of fitness or an advanced trekker. you can check the itinerary of 2D1N Summit here which is start from Sembalun and finish in Sembalun as well.
For a beginner or who is not a good level of fitness, they can do the 2D1N Crater which is started via Senaru and ending in Senaru. why is it recommended to visit the Senaru crater rim? as it is popular with the views, it is also called as the second summit of Rinjani, you can see the three Gilis and the best point to see the Segara Anak Lake.
What is the conclusion?
For an advanced who wish to complete the trek in the shortest time, you can take the 3D2N Full Trail
For an advanced who has a limited time, but wish to reach the top, you can take the 2D1N Summit via Sembalun
For a beginner or who not a good level of fitness and has a limited time, but wish to see the best part of Rinjani, then take the 2D1N Crater
For a beginner or who not a good level of fitness, but wish to complete the Rinjani trek, then take the 4D3N Full Trail
For a beginner or who not a good level of fitness and has a limited time, but wish to see the best part of Rinjani and visit the Lake and Hot Spring, then you can take the 3D2N Lake via Senaru.
Unofficially, Rinjani can be reached by several routes which are not recommended due to safety reason. this route is: