Can't really think of an all-time favorite, but here's my current top 5, in no particular order.
(Judged both by song and OP animation)
Minori Chihara - Kyoukai no Kanata
nano feat. MY FIRST STORY - Savior of Song
Livetune adding Fukase - Take Your Way
Tamura Yukari - Fantastic Future
don'twanttotypeallthenamesbutHidamariSketch - Sketch Switchl
(Judged both by song and OP animation)
Minori Chihara - Kyoukai no Kanata
nano feat. MY FIRST STORY - Savior of Song
Livetune adding Fukase - Take Your Way
Tamura Yukari - Fantastic Future
don'twanttotypeallthenamesbutHidamariSketch - Sketch Switchl