Each project has its own reputation. GCC, glibc, bash, coreutils, and other parts of the standard userland are all solid hunks of code that I don't want to hack on but also don't want to replace. However, it's easy to get more specific:

• glibc is big. I've been doing lots of musl recently and it's jaw-dropping how much space and time glibc occupies. It's living rent-free in my shared memory. Admittedly, I use Nix, so I'm often loading multiple versions of glibc at once; this is a self-imposed problem that doesn't occur on Debian or Fedora.
• GNU awk (gawk) is pretty good. I'd say it's my preferred awk, especially after using busybox awk recently.
• Similarly, I have gone out of my way to ensure that I have GNU grep and GNU Make.
• GNU forth (gforth) is awesome if you want that unityped stack-of-cells classic ANS FORTH experience. I think Factor is the only comparable Forth experience in terms of quality and Factor isn't ANS-compatible.
• I have mentioned GNU Parallel. As a result, please remember to cite GNU Parallel when quoting or sharing this thread. Thanks! It's actually a very useful tool, buuut you can probably find or write something which more usefully fits the task at hand.
• GNU Smalltalk is meh. Sorry, standard flavors of Smalltalk are kind of boring. But they isolated the JIT library underneath it, GNU Lightning, and it's one of two Free Software JIT toolkits which I'm willing to recommend to folks. Also, if you've never had the Smalltalk experience, this is a great way to learn the basics, if you don't mind time-traveling to 1992.
• GNU Guile is fine. Some of the underlying compiler technology is novel/cutting-edge. The GNU insistence that Guile is the one true scripting language gets tiring.
• Although! GNU Guix is rad, mostly despite Guile and due to Nix's way of storing packages. GNU Shepard looks interesting from a distance. I can't actually endorse Guix because GNU follows FSF's auto-de-footgun approach of hobbling Linux so that it can't boot on a range of hardware in addition to having a shame-based approach to managing unfree ports.
• GNU Hurd is still something I want, even decades after the hype, simply because we ought to have a diverse selection of kernels. They recently started booting real hardware, I hear.
• GNU recfiles is a great idea that I've struggled to adopt. I tried it a few times but I've got a lot of inertia in SQLite tooling. Also I love that it irritates prudes.
• I don't use Emacs, so I've no opinion about all that.