Nethack suffers somewhat because the era in which it was birthed required a lot of portability, so there is a ton of #ifdef-ery which can be difficult to reason about.
#ifdefs are themselves spaghetti; windows.c [0a] and files.c [0b] have tons of platform-related ifdefs (but, mercifully, not too much nesting), hacklib.c [1] has some deep-ish ifdef nesting (though, again mercifully, well commented)
Ok on re-reading some of this, I guess it was worse in my mind than it actually is. Nethack was the first large codebase I ever made any changes to and tried to understand, so maybe the relative enormity at the time made a negative impression.
Check out the source for Brogue [2] for what I consider to be pretty readable game code.
[0a] https://github.com/NetHack/NetHack/blob/NetHack-3.6/src/wind... [0b] https://github.com/NetHack/NetHack/blob/NetHack-3.6/src/file... [1] https://github.com/NetHack/NetHack/blob/NetHack-3.6/src/hack... [2] https://sites.google.com/site/broguegame/