I would recommend any (K)(X)Ubuntu 12.04 LTS.
Below is a little more nuance:
Linux Mint is build upon Ubuntu. This splits up between Ubuntu and Mint on one side and fedora on the other side.
Ubuntu 12.04 is a LTS - version. It will be supported for 5 years. So you don't have to worry for distribution upgrades for a while.
Fedora 17 is at this moment in beta stage and the current Linux mint is based on ubuntu 11.10. If you would opt for Linux Mint I would recommend you to wait for the version which will be based on ubuntu 12.04. Typically Linux Mint release is two months "behind" the ubuntu release.
Fedora is using more cutting edge technologies than Ubuntu and Mint, and is directed more towards advanced users. Fedora is more principal on using "free" software (free as in speech). This means that fedora is a bit more reluctant to install "closed" software (for example flash, or a nvidia proprietary display driver, multimedia codecs). It is not difficult, but you'd have to to a bit more effort to have your pc ready for all uses that you might have taken for granted in the past.
The main fedora is using gnome3. I'm not a huge fan of that. Fortunately they provide also KDE, XFCE and LXDE spins. The current XFCE 4.8 is already in Fedora 16. Although it is not at the level of gnome2 yet I like it alot. There are a lot of other spins in Fedora:
http://spins.fedoraproject.org/
Ubuntu is targeted towards giving the best everyday experience. It is not cutting edge like fedora, but it gives you the option to install the necessary "closed" software ready when you install the system. Also ubuntu has the largest user base among the linux distro's, which results in a stronger support forum and more things working out the box.
If you don't like Unity try XFCE (Xubuntu).
With Linux Mint you have the advantage that at installation you don't even have to select the option to have the "closed" software installed. A lot of stuff will be working out of the box. However, I find ubuntu always of a higher quality level than Mint.
If you mean by security being safe against malware, virus or other threads I don't think the distros will differ a lot from each other. If you want to be more sure of your privacy whilst surfing Linux Mint is using duck duck go in stead of google search as default. If you want to do penetration testing (or hacking) backtrack is the distro to use. But I would not recommend using back track for everyday usage.