I'm only an anime tourist for now, but I'm thinking of picking up the games when their respective seasons come around. Especially Kanon since I foolishly watched it when it's almost summer kek.
Anyway, out of the original Key trinity I liked Clannad the least because of the deus ex machina at the end.
Air I also felt didn't live up to its potential. The present and past events feel almost entirely disconnected. If you squint hard enough you can see Misuzu and Kanna have similar names but nothing came of it in the end. The girl really just died to unexplained ligma.
So that leaves Kanon. While I don't think it's perfect, the low points didn't bother me as much as those in the other two. I found the whole cast likable as well (Mai a bit less so, she does lack character).
They really knocked the atmosphere and music out of the park with these games though. Godlike OPs and EDs. Some of the BGMs play in my head when I go about my day as well. I miss the early 00s bros