What is reliable? I need reliable Android, to install "from zero", cleaning all the old firmware of the old smartphone.
... So, the first and main step is to download a reliable and complete set of firmware files: where the reliable domains if there are no "official source"? Where the reliable tools to help in the (ideally simple and plug-and-play) installation process? Where the reliable community (suppose here!) to help to select what is reliable?
As Wikipedia noticed as Mobile Malware,
As wireless phones and PDA networks have become more and more common and have grown in complexity, it has become increasingly difficult to ensure their safety and security against electronic attacks in the form of viruses or other malware.
And, in nowadays, the most commom and danger type of attack is against old-Android (with discontinuing support), malware that install itself in the Android operating system, so infects the so-called firmware.
The only two 100% reliable way to avoid after detected contamination is to pay for a new smartphone or to reinstall all firmware... The the only cheap option is to reinstall by yourself (supposing that to pay for reliable certificated service is the cost of the old smartphone).
The old or infected Android must be replaced by a new reliable Android operating system.... The so-called "firmware flash process".
There are no direict solution to the problem... So, the best that we can do is to votate.... This question is a voting pull about the main options:
...after a lot of research I discovered that there is no magic, no easy and super-reliable way to reinstall all firmware by myself, but there are two (or perhaps more) main options. Please post your answer as Wiki.
Illustrative example and scenario
Even for generic answers, to avoid long and abstract discussions we can use a typical smarthphone as reference. Suppose a Samsung Galaxy S-III GT I9300.
In this device, after hard reset, there are options for 1.
reboot system, ..., 3.
apply update from external storage, 4.
wipe data/factory reset, ...
Suppose that I used the options 4 and 1, but the virus persist. Now I need to use option 3, but the main problem is "where the reliable Samsung S-III downloads"?, there are no official download in the official domains like