Which cyber threat datasets have actually stood the test of time?

Every CTI team has a few datasets they rely on and dozens they've abandoned.

Which open or commercial datasets have consistently remained useful over the last 3–5 years?

I'm particularly interested in:

• low-maintenance data sources
• datasets with transparent collection methodologies
• sources that unexpectedly became valuable for long-term trend analysis
• datasets that looked promising but eventually proved unreliable

Concrete examples are much more valuable than rankings.