This is a difficult question and a personal one. We usually recommend the Machame, Marangu or Lemosho routes, these offer less crowds and more beautiful scenery. It is very manageable even with little experience, provided an honest effort is put forth in training.
As for the Marangu route, it is the quickest route and the one where you sleep in huts instead of campsites. It is very commonly called ‘coca-cola route’ because it is the easiest one but acclimatization needs to be done carefully and, if possible you should climb it in 6 days (instead of 5) and sleep one night or two in Arusha/Moshi before the trek.
Finally, Umbwe route is a spectacular and fantastic route as well, but only recommended for more experienced climbers.