Using information from Comichron, these appear to be how the Bat-related titles did for September 2015 as reported by Diamond Comics Distributors (http://www.comichron.com/monthlycomi...5/2015-09.html):
3rd - Batman 44 - 114,409 copies
7th - Batman Annual 4 - 80,684 copies
15th - Harley Quinn 20 - 56,836 copies
16th - Detective Comics 44 - 55,871 copies
17th - Harley Quinn Road Trip Special 1 - 51,135 copies
31st - Batman Superman 24 - 40,132 copies
37th - Robin Son of Batman 4 - 38,162 copies
45th - Harley Quinn & Power Girl 4 - 36,031 copies
52nd - Grayson 12 - 32,284 copies
55th - Batman Beyond 4 - 31,202 copies
57th - Batgirl 44 - 30,654 copies
81st - Grayson Annual 2 - 25,470 copies
91st - Black Canary 4 - 23,678 copies
94th - We Are Robin 4 - 22,893 copies
99th - Red Hood Arsenal 4 - 22,113 copies
104th - Batman Arkham Knight Genesis 2 - 21,555 copies
105th - Catwoman 44 - 21,507 copies
115th - Batman Arkham Knight 8 - 19,641 copies
124th - Batman Arkham Knight Annual 1 - 17,399 copies
126th - Gotham Academy 10 - 17,292 copies
136th - Scooby-Doo Team-Up 12 (with Harley & Ivy) - 15,436 copies
149th - Midnighter 4 - 14,431 copies
151st - Batman '66 27 - 14,315 copies
154th - Gotham By Midnight 9 - 13,698 copies
159th - Bat Mite 4 - 13,040 copies
Which, if any, have you bought as single issues, whether in print or digitally? (NOTE: All listings here are for print versions, even if the title is Digital First.)