"Kimi no Shiranai Monogatari ()" by supercell; performed by nagi (Gazelle)
used as ED for episodes 1 - 15 in Bakemonogatari; used as ED for episode 11 in Monogatari Series: Second Season.
Now, don't get me wrong. The openings in Bakemonogatari are also very good. There are 5 openings, for 5 different arcs, each sing by its own respective female lead actress in that arc. I especially liked the first opening, sing by Senjougahara's voice actress (tsundere to the maximum, in animation, lyrics, and the opening song itself).
However, I vaguely recall that I had a criteria about the OP and ED being used in as many episodes as possible. Well then, ladies and gentleman, here comes the ending, "Kimi no Shiranai Monogatari" by my favorite band (as aforementioned), Supercell!