In short either one. Now as far as 13.04 goes, support on it will expire on 01/2014. By then 13.10 will be completely out. If you want Ubuntu 13.04, you can download it Here. For installation instructions see Install Ubuntu 13.04.
Now, for 12.04LTS (Long Term Support), support on that will expire on 04/2017. If support is what you're after, then 12.04 is the one for you. You can download it Here. For installation instructions see Install Ubuntu 12.04.2 LTS.
So who is the latest version for? Well, if you want to be on the
bleeding edge, have the latest versions of all your software, and use
the latest features before they make it to the LTS version of Ubuntu,
upgrade to the every-six-month releases. If you’re a developer who
needs the latest versions of certain packages, you might want to
upgrade if getting them on the LTS version of Ubuntu is too much
trouble. If you use Linux because you like tinkering and experimenting
with the latest software — and don’t want things getting too boring
and predictable — upgrade to the latest release.
However, you’re not missing out much by using the LTS release. You
don’t have to upgrade every six months anymore — Ubuntu’s LTS release
is well-supported and will run all the software you depend on. It’s
regularly updated with new hardware support and performance
improvements, so you shouldn’t have to upgrade to make your Wi-Fi work
properly or dramatically speed up your desktop.
Source:HowToGeek