This one is a tough one to get one's head around. The simplistic shonen formula series are there, but you also liked Ai Baby. I'll get to that in a minute. Lucky Star, Azumanga, and Ouran makes me think of a few.

Seto no Hanayome (just ended), Mahoraba (a few years ago), Nodame Cantabile (two seasons back), and Honey and Clover (seasons 1 and 2). The art in Nodame and H&C takes some getting used to, but the second two are right up Ouran's alley (without the yaoi parody), and Seto no Hanayome is really crazy. Mahoraba is more on the heartwarming side, but it's still an "eccentric side characters" series.

First thing to pop into my head thinking of Ai Baby, AIR, and Kanon is Bokura ga Ita. A very realistic approach to a romance series. Next is Asatte no Houkou More of a fantasy element to it, which fits well with AIR and Kanon. Then I've got to say ARIA. A slice of life vein that gave me an even more tranquil feeling that Ai Baby gives off.

You suggested Fate/Stay and FMA, so I'll reply with Chrono Crusade. A slightly older series, one I think a lot of people newer to anime haven't seen, but definitely should. You've been around a while, but a lot of people forget about it.

These others don't fit quite so well, but I'll just throw them out. Try Suzuka (bear with it, she is a bitch), Paradise Kiss (I'd suggest Nana, but this is much shorter), Kamichu!, and Uta-kata (if you like Asatte no Houkou).