Simplest: Assuming you don't want to venture into stop motion post processing then I recommend the iPhone's camera app in Time-Lapse mode.
Medium:If you don't mind the post processing of stop motion then you can use any camera you have. If you are purchasing a camera for this I would consider multiple other aspects before making the purchase. Timelapse is often exported at resolutions that are far less then even the least expensive camera can output. That means you could get the cheapest possible camera and still make excellent time lapses.
Advanced: If you really want to delve into the fun of time lapse photography I would recommend an older Canon camera that is compatible with Magic Lantern, like a Canon 60D or 6D. It's a safe software you install easily onto the camera that enables a lot of stop motion functionality. It's a blast to play with and gives an older camera a TON of state of the art functionality.