Hm. Well, when Air was actually 'airing' I remember watching episodes of the show by Koi, Ishin, Lunar and the Triad. Out of those, Lunar was the hardest to watch, followed by the Triad - I just don't like the English that they used. Mind you, it has been a while - and I ended up getting the Summer Specials by the Triad for reason of expediency.
Koi provided for me the best balance of a very readable English sub, with hyper fast translation and quite decent quality. If I wanted more detail or clarity on any issue, I had but to hit this forum, which was, back in the day, absolutely huge. I very rarely needed too though.
-Andiyar