I couldn't tell you how well any of the Key anime did as adaptations as I haven't read any of the original source material. Hooray for raw expectations.
Angel Beats!--1st title. Maybe watching this without Clannad/After Story or K2k6 to inflate my expectations was a good thing because I was able to enjoy the show for its comedy. Its only faults are some plot points introduced when the show was being serious. It doesn't develop everyone but ubiquitous effort is the push, not the standard; anyone meant to be important gets their time to shine well enough.
Clannad--2nd title, 1st watch. It fell flatter than Fuko's chest.
Kanon (2006)--3rd title. The emotional investment they put for each character overrides each arc's self-contained nature. It also helped that the transitions after every conclusion were subtle; it's hard to guess who Yuichi is going to be with next while Tomoya waves a Kotomi or whoever flag at the outset.
Air TV-- 4th title. I barely remember the other heroines and lol Japanese Myth shenanigans at the end. The ending from a pure storytelling viewpoint is fantastic for not doing what so many other Key anime are subject to. Only from a pure storytelling standpoint though.
Clannad After Story--5th title. Develop some characters nobody gives a second thought to and you get a surprisingly strong first half of this anime. Repeat again for Tomoya's development in the second half and summarize the entire series from season 1 in a single episode. Then deliver a terribly written ending. Jun Maeda had the chance to create a masterpiece and he blew it.
Clannad--2nd title, subsequent watches. The show was always thematically unified and the musical score is very good. Hindsight makes a big difference here.
Little Busters!--Would-be 6th title. I'll Refrain from talking about this until the second season is up.
YEEEEEEEEEAAAAAAAAAHHHHHHHHH