Statistics pages, hands down. Not even close for return on time invested.
Pick a topic in your niche with decent search volume around "[topic] statistics" - target pages where the ranking content is stale (2022, 2023 data) or thin. Build a superior version: fresh numbers, original survey data if you can pull it off, or a far more thorough roundup of cited sources.
Then outreach to anyone linking to the outdated stat pages. Use Ahrefs (or whatever you prefer) to pull backlinks of the top five ranking entries, filter for editorial links, and pitch yours as the updated resource. Response rate absolutely crushes cold guest post pitching - you're solving a real problem for them (a broken or outdated source).
The last one I did netted over 40 links in six months on autopilot after it started ranking. Caveat: works best in B2B, finance, or health where people actually cite stats. Less effective in lifestyle niches.