I would use either Mint or one of the Ubuntu LTS releases. The Ubuntu releases in between the LTS ones can be a bit experimental and annoying. Mint's user interface is easier if you're used to Windows. The only thing less nice about Mint is that they don't officially support upgrading in place - they recommend that you wipe the system and do a clean install to upgrade, which is a bit silly. But you can upgrade Mint in place by editing your sources file and running an apt command, and in practice I've never had this cause a serious issue (well, no more serious than the issues Ubuntu upgrades cause). So, if you want an easy UI go with Mint, and if you want an easy upgrade process maybe Ubuntu LTS (but the Mint upgrade process is really quite simple once you get used to it).