Which Linux distro is best for software. I'm looking for one that has a large repository, good software, and a easy to use package manager.

Debian or Ubuntu offer one of the largest package repositories - though with any Linux system you can always build things from source if you want.

Arch has a decent amount of software, especially when you include the AUR (which requires more fiddling around, but not really that much in the scheme of things).

But Ubuntu is the most widely supported, and easier to pick up.

I agree with this. though another easy to use one would be Fedora. If you do go the Fedora route though make sure you grab somethign other than the GNOME3 version as the UI will degrade your productivity. And as far as the AUR goes it isn't too bad once you do a couple of them. It is just a few lines in a terminal, at least for the couple of packages I installed from there.