Which is the best Zigbee application is not a simple question that can be easily answered as it depends on each individual user’s need and experience requirements as well as which specific Zigbee devices you have and plan to buy. They all have different degrees of integration with Home Assistant and different levels of support for some devices.
- ZHA is the easiest to get started with and the only one natively integrated into Home Assistant. No need to know anything about MQTT, but not as advanced or flexible as Zigbee2MQTT. Recommended as plug-and-play for beginners who buy standard Zigbee devices.
- Zigbee2MQTT has the largest amount of contributing developers usually the first to get full support for odd devices that deviate from Zigbee standard and require custom converter code for quirk translation. Not a native integration so devices are exposed via MQTT. Recommed for advanced users who require Zigbee configurtion customization.
- Phoscon/deCONZ works and integrates like a third-party gateway/hub similar to a Philips Hue Bridge. No need to know anything about MQTT. Closed source / proprietory application so not much users can do to contribute other than reporting bugs.
Remember that as mentioned you can run all of these at the same time if you wish as long as they each have their own separate adapter. Suggest that you also buy an a couple of inexpensive CC2652P adapters as well so have option to test others. Note that you can also reflash CC2652P adapters with Zigbee Router firmeware and reuse as a repeater for range extending.