Best laravel version to have with existing php ?

I recommend you to upgrade it to the latest version (5.7): https://laravel.com/docs/5.7/upgrade

But it depends on your php version in your server. You can see the requirements for the different laravel versions here (in the top right you can change the version and see requisites): https://laravel.com/docs/5.7/installation#server-requirements

@MANELGAVALDA - i forget to say that this is the answer i got : Current PHP version: 5.6.29-0+deb8u1

5.4 requires php 5.6.4.

5.5 has 7.0 as requirement so laravel 5.4 should be good to go.

@YOUSSEFBOUDAYA - The last one that your php version supports, is the 5.4 (https://laravel.com/docs/5.4/installation). But I recommend you to upgrade your current php (5.6) to the current stable version of php(7.3.x).

Which host did you have that cannot run on PHP 7.x ?

@MANELGAVALDA - this is the first time i'm trying to upgrade a laravel project and i'm used to working with 5.4 version(almost 2 years).this project is not created by me so i think it would be better to upgrade to 5.4 because in case i get error or bugs i think i'll be able to solve them.

Just be aware of that : http://php.net/supported-versions.php

5.5 is Lts.