We're in 2018 and some nice guy wrote an awesome python script which autoselects the best mirror based on the country you provide and the latency to the servers AND prints you a 'sources.list'. You can simply take this new list and replace the one in '/etc/apt' with it as it's an exact replica except for the updated hostnames.

To install the tool called 'apt-select' you first need PIP the python package manager:

$ apt install python-pip
# or
$ apt install python3-pip

Once installed, run the following command to get the top 5 fastest servers near you. Replace "DE" with your country:

$ apt-select -C DE -t 5 -m up-to-date -c
# -C Country
# -t Top
# -m minimum-update  
# -c choose from top and create sources.list

Simply overwrite your current sources.list with this one:

$ sudo cp /etc/apt/sources.list /etc/apt/sources.list.bak
$ sudo mv source.list /etc/apt/sources.list

Now refresh your cache from your new mirror:

$ sudo apt update

Old Article: To speed up package downloads we can add the following to our sources.list and apt will automatically find the fastest mirror for us. Note: If youre running a different version of Ubuntu replace 'xenial' with your distributions codename. (DISTRIB_CODENAME in /etc/lsb_release) The following command automates this:

sed -i.bak 's/.*xenial.*/deb mirror:\/\/mirrors.ubuntu.com\/mirrors.txt xenial main restricted universe multiverse\ndeb mirror:\/\/mirrors.ubuntu.com\/mirrors.txt xenial-updates main restricted universe multiverse\ndeb mirror:\/\/mirrors.ubuntu.com\/mirrors.txt xenial-backports main restricted universe multiverse\ndeb mirror:\/\/mirrors.ubuntu.com\/mirrors.txt xenial-security main restricted universe multiverse/' /etc/apt/sources.list

Then update the repository cache using apt update.

Legacy Method