The latest hardware is OS 3.0 (either AC or DC). The main difference of OS 3.0 with previous versions is that OS 3.0 has built-in WiFi (but no wired Ethernet port any more). Also, it supports over the air (OTA) firmware update (through WiFi) and no longer requires USB cable to upgrade firmware. The firmware running on OS 3.0 is currently 2.1.7 and is completely compatible with previous OpenSprinkler. You should be aware that OS 3.0 uses different expansion board than previous versions — so if you need expander, OS 3.0 must be used with Expander 3.0 and cannot work with Expander 2.0 or 2.1.
Additional details can be found in the Getting Started Guide on the product page: