My pick isn't technically a series finale, but it is one in my heart, god damn it.
S03E16 Sleigh Ride, the season three finale of Fear the Walking Dead. It was the final episode under original showrunner Dave Erickson— beginning with season four, he was replaced by the guys behind Once Upon a Time. They wanted to (and did) take the show in a very different direction than Erickson had wanted to, so he had to wrap up several things that he had originally intended/wanted to let play out much longer.
The thing that sucks is that when it came time for the new showrunners to continue the story and pick up the threads that Erickson did leave for them, they just… kinda chose not to? A few of the characters return, but two of them are killed off* within the first eight episodes, and the show puts a larger focus on a new cast that's introduced. What's especially weird is that one character returns despite having already been written out of the story early in season three, and their return is barely addressed— and they're given very little to do in the new era of the show, so their return just seems pointless. In addition, the season three finale ends with a sort of cliffhanger, and they kind of just handwave it away in season four. The main villains of the second half of season three are never even mentioned or referenced again after that season's finale, and their fate is left up in the air. They just got ignored by the new writers.
The season three finale may as well be a series finale of the "real" Fear the Walking Dead. What came afterwards is Fear in name only, it's very different. More lighthearted and corny, tropey, not very grounded, etc. They've tried to capture the original spirit of the show at times, but they always end up going back to very comical things, and the writing is generally poor. It's disappointing.
*One of those two characters' deaths has been retconned now (in season seven), and they're returning later this season. But said character was originally intended to very much be dead, and for all intents and purposes, they have been for the last few years.