Carpet is the least expensive flooring to install and to replace in a rental unit. A brand new carpet will attract a new renter. Most renters do not take care of the floor, so when they move, carpeting can be easily ripped up and quickly replaced for the next renter.
Rental units tend to be high traffic and high turn over types of living arrangements. I would suggest using something that is durable, but not very expensive because chances are you are going to be replacing it often. Some flooring options that come to mind are laminate flooring, vinyl coverings, tiles, berber carpeting, etc.
You want to have something durable, easy to clean and not to expensive to install and repair. The type of flooring also depends on whether it's a kitchen, a bathroom or a living room. You could put vinyl or linoleum tiles in the kitchen, ceramic tiles in the bathroom, and hardwood in bedrooms and in the living room.
If you are the landlord and plan to rent the unit out for a long period of time, use a composite that resembles hardwood, which is cheaper and more durable than hardwood. If you are a tenant, you should not alter the flooring without express permission from your landlord.