While this is a nice collection of tools, to which I'd add the KDE / QT Filelight, which is similar to other gui disk space usage tools, it's not the literal answer to the topic title.
If you take the question of this thread literally, that is, what are the largest packages in your system, as a rule, they are:
The libreoffice suite of packages, which isn't a single package, but it's really big.
The linux kernel and associated header packages comes I think second, and if you have multiple kernels installed, which most systems tend to have, each one is around I think 135 mB disk space, roughly, gets bigger every year, but it's about that big. If you use non free video drivers there would be the associated driver packages/files as well. Note that as with libreoffice, each kernel actually can actually be made up of 2 or 3 packages, the image, header, and driver packages, so a literal per package size count isn't actually that accurate except to get a crude sense.
After those, there aren't a huge number of massive packages in Debian's package pool that I'm aware of.
I find that once the root partition starts getting big enough to where you start wondering how to get rid of packages to shrink it, it's generally time to move/resize to a bigger root partition.
apt-get clean of course is the first thing to run, to get rid of all the cached .deb files, but once you remove that, and all but say 2 kernels, if you're still stuck with not enough room, it's more practical to increase the partition size since the cleanups etc just are red flags you are running close to out of root partition space, which can be a real problem if apt doesn't have enough room in /var to actually download and extract packages for an upgrade. Cleaning out by purge actions leftover config files from removed packages won't do you any good at all in terms of regaining disk space since they are tiny, but it is nice to do now and then just to clean up cruft, but it shouldn't be mistaken for actually regaining any meaningful disk space.
apt-get auto-remove isn't bad either, but be aware that it's sometimes wrong about the package being removed, so you want to check that list carefully before you let apt remove the stuff. Technically it's better to purge that list, that way you get rid of the config files as well at the same time, remove or auto-remove will I believe leave them in place.