I can advise you to seriously consider Blockstream's Jade hardware wallet as it has all the code clearly visible on Github and a very low cost of around $70 or so.  And that's not all, you can also compile this wallet on an esp32 card such as the lygo or stick c plus and reduce costs to just a few tens of dollars.  Furthermore, I had already talked about this choice in the thread opened a few weeks ago. 
https://bitcointalk.org/index.php?topic=5450647.msg62169741#msg62169741

While if you ask me which is the best software, my choice falls on Electrum.