First of all, according to my current definition, the Charlotte anime is a key work, the Angel Beats anime is a key work, Little Busters and Clannad visual novels are key works. Their anime adaptions are not key works - they’re adaptions of such.
The reason I like the Angel Beats anime way more than Charlotte is pretty simple: Angel Beats completely blew me away. It gave me tons of feels. I didn’t even know a work of fiction was capable of eliciting so many emotions of that intensity. It was what got me into Key in the first place. Angel Beats is just really powerful. It’s incomplete, with a big chunk missing right in the middle, but the parts parts of it that we did see were doing their job wonderfully. Also, I consider AB’s setting to be the ultimate emotional setting, something Key arrived at after working on many masterpieces. A place where children’s souls arrive at after a short life of struggle and an unfair death. With just that, there’s an immense potential in the story, and even the incomplete AB anime made good use of it. Even with all its faults, the AB anime is still up there, right next to the other Key works.
Charlotte didn’t have that kind of impact to me. It didn’t leave all that much of a lasting impression compared to any other Key story. It didn’t leave behind as many fond memories. Even in terms of music: Angel Beats has “Ichiban no Takaramono”, Little Busters has “Haruka Kanata”. Memories and feels start flowing into me as soon as I hear those. No part of the Charlotte OST will be that kind of “trigger” for me. The most memorable piece would be the opening.
Now don’t get me wrong. I’m not blaming Charlotte for not being dramatic or emotional. It never tried to go all-out on drama. The thing is, it just didn’t leave any other comparable kind of impact, either.
So for these and several other reasons, I will always like the AB anime way more than I did that of Charlotte. It’s not an answer I arrived at after sitting down and counting the number and gravity of mistakes that the respective anime have made or anything like that. Those aren’t all that important to me.