AIR is my favorite story. I originally wanted to talk about it in the favorite Key topic but my thoughts became too long so I’ve decided to put them here instead. Sorry if my writing isn’t the best here but I have not really the time to do better 
AIR was an improvement over nearly everything compared to Kanon for me.
Kanon totally changed my life and I absolutely loved it but it was still a flawed game and much of the routes were not that good for me. AIR has a main story that is cut in 3 routes which becomes better and better and Minagi’s route is one of the best Key routes I’ve read, so the only bad part from it is Kano’s route (and even then it’s still not the worst Key route I’ve seen).
Kanon had a wonderful wintery atmosphere and AIR succeded to have a great atmosphere as well in a totally different way (summery and isolated ambiance). AIR is extremely poetic, it has a seaside town, dreams, sunflowers, a girl in the sky, angels, traditional japan, the moon (Summer), the starry sky (Minagi), … Just compare the arts/fanarts of AIR and CLANNAD, you’ll see that the arts of CLANNAD are really plain compared to the arts of AIR that are absolutely beautiful, AIR has many elements of beauty and the artists love that.
AIR is extremely original and has a lot of good ideas. It is not a traditional high school slice of lice like Kanon and CLANNAD, the MC is a vagrant that tries to gain is life on the street and not a high schooler having a fairly easy life. AIR really has a main story (instead of a cover girl route) divided into 3 parts, which was a novelty at the time, and the second part takes place 1000 years before the first one (it blew my mind how the changes of musics, interfaces, way of talking, … felt like reading a totally different VN all of a sudden). And there were also things like the uses of the map to move around the town or the beginning of AIR where we live the story again through the point of view of a bird (I’ve seen that better done in other stories since then but it’s still really original).
Yukito is an awesome protagonist, it just feels good to incarnate him and I don’t know a lot of Key MCs that would have the balls to sacrifice themselves to save the heroine. This is my favorite scene of the anime by the way, Misuzu and Yukito are such a great couple and we can feel all the love that they have for each other in the last scene of Yukito. After that, Yukito reincarnates as a bird and he still tries pathetically to help as much as he can the girl he loves. What he has done is essential to the story, he prolonged the life of Misuzu enough for her to live the AIR part of the story and he weakened the curse enough for Haruko to not be affected by it and for it to be broken at the end of the story (even if Haruko played an important role too to break the curse, as an old character of the Key writers said “Miracles always stem from the bonds you share with others”).
I’ve talked a lot about Yukito but the whole main cast is amazing too and I think it’s interesting to point out that the 3 special top 3 of the last Key popularity contest included a character from AIR (for best MC, best mom and best mascot). I’ve seen some people arguing that Sanae is the best Key mom and it’s true that it would be a dream to have her as a mom, however as a mom character, Haruko is far better since she has way way more character development, she’s the protagonist of the AIR route with Sora. I think Haruko is close to Yukito, she’s basically the opposite of the perfect mom (that we can find with archetypes such as Akiko or Sanae), I’d love to have her as a mom though, it would be a lot of fun to drink with her.
And I can’t talk about my love for AIR without mentioning Misuzu.
Yukito and Haruko are amazing characters in their own departement but I don’t think Misuzu is necessarily the best Key heroine (although I’ve no idea who would be the best objectively). However she’s still an amazing character. Her courage and her determination are admirable, no matter how crappy her life gets, she never gives up. We can see that right in the beginning of the game where she tries her utmost to be friend with Yukito. After that, things don’t go well to say the least, but at the end she still managed to do the final steps in order to achieve her goal. The things I live with a character in a Key story are an important part of my appreciation for them (see Makoto in Kanon) and boy, it was really a ride with Misuzu. If I was with her I’d try my utmost to make her happy during her whole life in order to slightly compensate for all the suffering she has experienced during her whole existence.
It’s through the common efforts of everyone (Haruko, Yukito, Uraha, Ryuuya, dozens of generations of people searching the girl in the sky,…) that Kanna/Misuzu was finally released from her eternal torment. I think, Maeda wanted to show this message of bonds more explicitly with the final scene where the two kids at the beach are being able to go to the future as long as they’re together (and the final picture/logo where a girl holds her hand with someone). Unfortunately, people didn’t like this abstract ending (which is even worse in the anime) and Maeda will say that way more clearly in CLANNAD.
Another important thing that says AIR for me is YOLO (it’s not technically true in AIR because reincarnation but that’s not the point ). Even if Misuzu had a terrible curse that condemned her to be alone, the characters didn’t care at all, Yukito and Haruko both leaved MIsuzu but they both came back to be with her despite the circumstances. In the end, Misuzu lived a really short life but she died in pure happiness, surrounded by her loved ones and that was way better than having lived a long and sad life alone.
While I’m at it, I can also talk about the MInagi and Summer routes.
Minagi’s route reminds me how much the supernatural aspect of Key stories can be good and creative. It just makes me sick when I think about all the crappy drama, in various popular media, that try their best to make us cry and fail miserably while Key uses way more original formulas and makes the saddest stories I’ve ever seen.
Yuuichi Suzumoto made an extremely clever and well made story with the Summer route as usual. Usually, most stories tend to have a very good beginning and end but a meh middle, that’s not the case at all with AIR because of Summer, the main story just gets better and better thanks to it. Summer is really a breath of fresh AIR (damn, someone already made this pun here) in the VN, it gives a lot of background to the story and a modest but still interesting universe (contrary to Kanon that was 100% focused on the characters), thanks to it, there are a lot of interesting things to say about the influences of Buddhism on the story.
So yeah AIR is awesome and the only thing I regret is its poor popularity in occident due to the anime which could have been better to say the least. Hopefully, one day we will have an English release of the game on Steam and on console in HD (and with the bonus route translated please) and everyone will see how great this story is 
Now that I think about it, I forgot to talk about best doggo, so to finish, I’ll just say PIKO PIKOOOOOOOO!!
