What Linux distribution has the most up-to-date packages?

Update: This answer does not help anymore.
As noted by Ethan in comments below the OSWatershed.org site has changed hands. There are multiple other places still pointing to that domain name like this answer does :-).

I would have deleted this answer however it is a good memory of such a site's existence. Maybe there is another similar site or someone might fork and update this one from its source -- (see https://github.com/tannewt/open-source-watershed).

Meanwhile, ArchLinux continues to be at the top.
Another place that might be useful: https://distrowatch.com/packages.php

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Look at the Open Source Watershed page.

OpenSourceWatershed is a project aimed at understanding the relationship between distributions (downstream) and the individual software components (upstream). It is the basis for a larger study of distributions and their evolution. It is distrology. In the future, more distro oriented statistics will be available. More details are below. For now search in the top right for your favorite package to see how up to date the different distributions. Or look at the right to see what new releases happened in the last 24 hours.

Arch Linux is presently at the top.

But, when you have a specific development project of interest,
I think, its better to track the source straight -- rather than depend on disto updates.

debian unstable

dozens, sometimes hundreds of new and updated packages every day. if you want bleeding edge packages, this is it.

there are some distributions that work with a rolling release schedule,meaning that when a certain piece of software is released,it is included in the distribution once tested and found compatible with the distribution

examples of those distros include Arch Linux,Gentoo and PCLinuxOS