I may be the odd-ball here, but I actually thought that the story in Halo 3 was the best in the three games, especially after the High Charity crashes into Earth.
But as for the number of people playing only the single-player = quality of the story, that doesn't mean a thing. NSMB has 19 M people playing the game and it's only single player, yet the story is plain, even in Mario Bros standarts.
Halo is halo, the best console FPS thus far, so that's the reason so many people play it, not the story per se.
Current PC Build
CPU - i7 8700K 3.7 GHz (4.7 GHz turbo) 6 cores OC'd to 5.2 GHz with Watercooling (Hydro Series H110i) | MB - Gigabyte Z370 HD3P ATX | Gigabyte GTX 1080ti Gaming OC BLACK 11G (1657 MHz Boost Core / 11010 MHz Memory) | RAM - Corsair DIMM 32GB DDR4, 2400 MHz | PSU - Corsair CX650M (80+ Bronze) 650W | Audio - Asus Essence STX II 7.1 | Monitor - Samsung U28E590D 4K UHD, Freesync, 1 ms, 60 Hz, 28"
