I'm looking for whatever box would have the best WiFi. If I would get better performance with a USB WiFi stick attached to it I'm open to that solution as well.
It'll be used mostly for IPTV and Plex so some good hardware codec support would be awesome too.
You are looking for devices with MIMO (Multiple Input and Output) WIFi plus multiple WiFi antennas as well.
External Antennas on small media players are usually always better.
You also want Dual band 2.4 / 5GHz AC WiFi.
You cannot just plug in a USB stick WiFi antenna and expect it to work with Android devices. In fact most times they will not work at all.
Dunno what you're expecting here there are basically two Android TV boxes:
Mi Box S - $60
Shield TV - $140 to $180 depending on sale
Both have good wifi. I've never seen a review testing their range though.
You will be better served putting together a better wifi network. No box can make up for poorly placed or really old access points.