It depends on the features you want. If you just want software that scans on-demand, but nothing else, ClamXav is a great free app from the App Store, and its rating on the app store speaks for its efficiency and simplicity. Since it has no constantly-running processes to protect the computer, it takes up very little memory.
However, if you want antivirus software that actively scans open network connections, mail, and files accessed in the file system, I find Avast! free antivirus for Mac to work pretty well. The web shield, as it's called, can mess up programs that need to download files (like Dropbox, for instance), but it still can be used.
From my experience, you probably won't need software that can actively scan your system. ClamXAV usually works very well, but if you really feel the need for more powerful software, Avast! is a good way to go. However, whichever way to go, neither has any ads and both are completely free. They also don't create tons of annoying notifications.