There are two standard editions of the Greek text of the New Testament. These are the Nestle-Aland edition (aka Novum Testamentum Graece) and the United Bible Society edition. These are in their 28th and 5th editions, respectively. The Greek text is identical in the two versions, but the apparatus is different. NA 28 contains more notes regarding textual variants, UBS 5 contains fewer notes, and is designed for translators.
NA28 - Novum Testamentum Graece
by
Nestle-Aland, United Bible Society
Tyndale House - Greek New Testament
by
Tynadle House, Cambridge
Here is a "User's Guide" to the Nestle-Aland Greek New Testament. This resource helpfully explains how to use the apparatus and get the most out of the critical edition.