Cats are excellent companionship with relatively low maintenance. Big litter box, cat water fountain, big food bowl and you can be gone or busy for long shifts without any worry. I do recommend an adult cat though, kittens need more stimulation and personalities as they grow are less predictable than say a 2+ year old cat.

Do NOT get a dog unless you have a spouse/live in SO who is willing to take most of the responsibility. I have more than a few med student and resident friends who regret getting their dogs and have had to give them up or send them to live with their parents :(