Short answer: the Asus ROG Phone 9 Pro — in independent lab tests it currently posts the longest runtime (about 20 hours 30 minutes in Tom’s Guide’s battery test). (tomsguide.com)

Quick alternatives by category:

• Best overall/flagship endurance: OnePlus 13 (≈19:45) and OnePlus 13R (≈18:49). (tomsguide.com)
• Best budget/longest cheap phone: Motorola Moto G (2025) (~18:32 in Tom’s Guide and top spot in some budget lists). (tomsguide.com)
• iPhone option with excellent real-world life: iPhone 17 Pro Max (~17–18 hours in recent reviews). (tomsguide.com)

Notes:

• “Longest” depends on the test (web browsing, video playback, gaming). Tom’s Guide’s lab uses a standardized web/browsing battery test — that’s where the ROG Phone 9 Pro led. (tomsguide.com)
• If you tell me which use case matters most (heavy gaming, video, all-day mixed use, travel), I can recommend the best pick for that scenario and where to buy it.