I have 2 Kodi STTBs in action currently, and one on the way.
I have Asus Chromebox acting as the main Kodi STTB at the moment. Its perfect for 1080p streaming, and does its job well. However, once I get a 4K TV going, this unit is getting replaced by a newer device to do 4K60 streaming.
I use a fairly old Intel Atom D510 based Zotac unit, which is now used to stream Youtube mainly in a bedroom.
I have an ODROID N2 unit on the way. This is a very recent device fully supported by Kodi/CoreElec and can easily stream 4K60 + HD Audio among many others. It has the AML S922 logical unit and its supposed to be top dog at the moment.
Not sure, what you are after. Apple 4K TV is also very popular, but it has certain limitations I noticed.
Another popular option is Nvidia Netshield, which can also do Kodi and has a lot of support.
There is a wealth of information on Kodi/CoreElec/OpenElec forums. You just have to see what suits you the best.