The production of Justice League was troubled, with original director Zack Snyder leaving part way in (for family reasons) and Joss Wheddon coming in to do extensive rewrites and re-edits. Although the final film is watchable, it's still not great in comparison to the filmic output of Marvel.
Buffy the Vampire Slayer (1997)
A second stab (pun intended) at Buffy produces far superior results that ran for an impressive seven seasons.
Starting off a bit slow, Marvel's SHIELD series has grown to be a dependable corner of the MCU, one that tells consistently good, deep stories as it explores pockets of the universe the films simply don't touch.