Which type of bird can mimic sounds and sing?

I'm looking for a pet bird who is good at imitating sounds and songs (such as ping pong balls bouncing and whistling a tune of a song), who is not overly large (largest size would be a cockatiel) and who can deal with a students day of class while being played with and let out in the afternoon/night time. I wasn't sure if a budgie or a cockatiel was more adept at the sound mimicking or if there was another bird who was superior to both? Thanks.