The Mac and Windows app VideoProc by multimedia software developer Digiarty is among the most comprehensive video converters out there. Not only does it transcode foreign video formats including MKV, DIVX, FLV and WMV to iOS-friendly formats, but also lets you convert videos in the High Efficiency Video Coding (H.265) format captured with your iPhone to the more compatible H.264 codec, which lets you play them back on older devices.
How to convert iPhone HEVC videos to H.264 with VideoProc [sponsor]