Comments and links about LTS systems
It can be difficult and confusing to select the version of Ubuntu both for the first installation and for upgrading to a new released version. This answer will focus on how to find the version with the longest remaining support time (until 'end of life'), which is often the first point release of an LTS release (LTS stands for Long Time Support).
Right now, when this is written, it is Ubuntu 16.04.1 LTS, and its iso files are not found via the 'usual' links for the current versions.
The support intervals are described with details and diagrams in this link,
The kernel series of the second, third and fourth point releases are different from the kernel series of the first point release, and are not supported for a long time. The hardware enablement stack must/will be upgraded according to these links,
in order to keep everything up to date (including security updates of the kernel). This is risky and I would discourage you to do that. Many failures are reported. Some of these problems can be found, if you type HWE into the 'Search Q&A' window near the top right corner of the web browser's window with AskUbuntu (and press Enter).
The kernel series of the fifth point release is that of the next LTS relesase and has long support.
Strategy for a stable and reliable system
I would recommend the following strategy, if you want to use a stable and reliable system.
A good backup routine for all files, that you want to keep, is the fundament of a stable and reliable system. This includes your personal files and the whole or parts of the operating system and installed application programs and tweaks.
Install and stay with LTS releases that are released in April during even years, 2014, 2016, 2018, ...
Install the version which has the longest time until end of life right now. Test it and if it works well, stay with it. "If it ain't broke, don't fix it"
Stay with update & upgrade within the kernel series
sudo apt update
sudo apt full-upgrade
When the next first point release is released, wait for a month, and sometime late in August, the worst bugs are fixed, and you can install the new release and do the updates & upgrades to a well debugged and polished system. This can mean changing
- from 14.04.1 LTS to 16.04.1 LTS, or
- from 14.04.1 LTS to 18.04.1 LTS (only standard Ubuntu has 5 years LTS), or
- from 14.04.5 LTS to 16.04.1 LTS, or
- from 14.04.5 LTS to 18.04.1 LTS (only standard Ubuntu has 5 years LTS), or
- from 16.04.1 LTS to 18.04.1 LTS, or
- from 16.04.1 LTS to 20.04.1 LTS (only standard Ubuntu has 5 years LTS), or
- from 16.04.5 LTS to 18.04.1 LTS, or
- from 16.04.5 LTS to 20.04.1 LTS ...
It is often better to install a fresh system and after that copy your personal files from the old system to the new system. It is possible to upgrade from the old LTS release to the next LTS release via
but it is risky, and you had better get a fresh backup before trying it.
Get the iso files
Start looking for the iso files of the version with the longest remaining support via these links,
If the iso files of the version with the longest remaining support are not found via those links, you can find them via the following general link,
and right now, when this is written, you want to find Ubuntu 16.04.1 LTS via the following link,