The 5.5-inch 6s, with optical image stabilization extended to video and the largest battery Apple shipped that year.
Released September 25, 2015 · Discontinued September 12, 2018 · From $299 (16 GB, with contract)

The 6s Plus took the 6 Plus body, rebuilt it in 7000 series aluminum, and added 3D Touch, a 12-megapixel camera with 4K video and the A9. Optical image stabilization now covered video as well as stills. The battery dropped slightly to 2750 mAh to make room for the pressure-sensitive display layer, though the more efficient A9 meant most owners never noticed. RAM doubled to 2GB across both 6s models, which mattered more than anything Apple put on the box: Safari tabs stopped reloading every time you switched apps. It shipped September 25, 2015.
Engadget reviewed the 6s and 6s Plus together and rated the pair on the same terms, with the Plus gaining optical stabilization.