Such a plant species whose research outcome will help the scientists & common people as well and I will also get plenty of interest to work on that species life long.
Easy names come through plants like money plant , dwarf bomboo, and many others since they bring so much energy and freshening atmosphere around you...that they are favourites amongst all classes of people..
It is not possible to live with a single plant species. Every and each plant have thier own importance. If you will want to try an experiment, you can select multipurpose trees like Sesbania grandiflora.
If it has to be narrowed down to a single plant species, then I will definitely prefer the one having the most diversified and multiple utilities, such as Azadirachta indica, Moringa oleifera, etc.
As per my knowledge it is very difficult to live with a single plant species. However if I am to choose I would say Potato. Because, it is a vegetable (source of vitamins and minerals) at the same time it also serve as a stable food by giving bumper yield per unit aria, time and inputs to feed the world.