Small binaries that employ a reduced set of programming/anti-debugging/obfuscation/etc techniques. Sometimes you have an idea what sort of techniques you're up against, sometimes you don't but you have a general idea of difficulty. In all cases the idea is that the possibility domain is bounded and smallish rather than completely unknown.

The idea is you conquer progressively more complicated crackmes and this helps you learn how to tackle random binaries you'll find in the wild.

Generally the step up after crackmes is to find virus payloads/samples and try and unravel those. I understand Stuxnet uses some of the most complex techniques to date: https://www.symantec.com/content/en/us/enterprise/media/secu... (ref: https://news.ycombinator.com/item?id=17099969, also https://news.ycombinator.com/item?id=2212922)