Short answer: Apple, Google (Pixel), and Samsung are the leaders — with slightly different strengths.

Summary of why:

• Google Pixel — best for the fastest Android updates and clear long-term guarantees. Pixel phones get day‑one Android releases and Google now guarantees long update windows (recent Pixel flagships have explicit 7‑year support; many earlier Pixel models got 5 years). (blog.Google)
• Samsung — matches Google on longevity for many recent Galaxy models and applies long support across flagships, foldables and some tablets (Samsung has publicly promised up to seven years of Android/One UI + security updates for many 2024+ devices). Samsung’s rollout timing can vary by carrier/market, but its public commitments are strong. (9to5google.com)
• Apple (iPhone) — extremely reliable in practice. Apple historically supports iPhones with major iOS releases and security fixes for many years (often 6–8 years for major updates), though its formal public minimum commitment is currently five years for security updates. Apple’s ecosystem and tightly controlled hardware/software make updates very consistent and broadly available to older devices. (arstechnica.com)

Which to pick depending on need

• If you want the fastest Android upgrades and developer-level fixes: Pixel. (androidauthority.com)
• If you want broad device choice with long guaranteed support across many models (flagship + foldable + some midrange): Samsung. (gizchina.com)
• If you want the most consistent, long‑lasting real‑world support and the largest catalog of older models that still receive updates: iPhone (Apple). (androidauthority.com)

A couple extra notes

• “Most reliable” depends on what you value: fastest access to new Android features (Pixel), the broadest multi‑device guarantees (Samsung), or the longest history of practical multi‑year support and uniform rollouts (Apple).
• Many other Android makers (OnePlus, Nothing, Xiaomi, OPPO, etc.) have improved their policies lately, but update lengths and rollout timing still vary model‑to‑model — check each model’s published update promise before buying. (indiatimes.com)

If you want, tell me which phone models you’re choosing between and I’ll compare their specific update promises and recent rollout reliability.