Oracle 18c is the latest version of Oracle in 2018. But it is available only on cloud and on Oracle engineered systems. If you want to use Oracle latest version on your system, then you can choose Oracle 12c. The latest version of Oracle 12c is release 2, which you can download from the following link click here.
Recently Oracle Apex 18.1 also released. If you are an Oracle Apex developer then you can check this post to how to upgrade with Oracle Application Express 18.1.
You can get more information about Oracle latest version from the following link click here.