Your opinions on good dog breeds please

I'm sure many of y'all are owners of dogs, I'm looking to adopt a dog from the local shelter and I was curious to know what type of dog that is at least medium to large size that is a great watch/security dog of some sort. I don't like small tiny dogs they annoy me for some reason. I will be living by myself during college and would like to have a nice companion as well as something that wouldn't mind being protective. Thanks for any opinions everyone !!