The version of Pixel you should choose largely depends on your preferences for Tag Integration and Setup.
V4 is considered outdated but works with Tag Integration and Tealium.
V5 only works with Tag Integration. With Pixel V5 you only need to integrate a special script into your website that is called "Tag Integration Loader”. This script loads and executes a Tag Integration Container which includes our tracking library, your tracking configuration, and optional tracking plugins.
SmartPixel (V6) does not work with Tag Integration. There is a tag in Google Tag Manager you can use. However, Smart Pixel can be less complicated, as it does not rely on your knowledge of Mapp Intelligence's data architecture. It can be particularly easy to use Smart Pixel if you need Single Page Solutions, as we have libraries for integration.