[malachi22]

Mediatek phones are the easiest to infect. Plenty of kernel vulnerabilities. I'd recommend by starting to learn about kernel hacking as you can do a lot more than with just standard root exploits. Also, if someone has an antivirus on their phone, it will pick up a patched apk in an instant. However, if you do /tmp or kernel exploits, they are usually undetectable on most Android devices (Samsung are an exception due to Knox)

Out of the hardest phones to exploit, snapdragon phones are by far the hardest. Followed by spectrum (just by sheer lack of information) and the easiest is Mediatek.