First, the answer to the question. Of everything we've translated, we're probably the most pleased with and proud of our work on Negima!.
There are a bunch of reasons for this, which is probably why the answer came so quickly, even if we weren't sure what the reasons were. First, the circumstances under which we were offered the title made it clear that we had earned the assignment. It's a really popular series, it's a comedy, and it takes a lot of time to finish. And the fact that we eventually received the assignment meant we'd proven that we could meet deadlines, and we were good enough at translating and adapting comedy that Del Rey knew they could trust us with the title and still avoid fan uprisings (due to delayed release dates and/or unsatisfactory translations).
Add to that the fact that, despite having about a million translators on the series before us, we managed to stay with the series to the end, and even get assigned to do new translations for the omnibus, and we feel pretty pleased and blessed that, not only did we avoid being fired, but we also avoided burning ourselves out on the series. (And some of those lexicons seemed to try really hard to get us to quit.)
But most of all, when we think about our work on Negima!, we really get a sense that we really do know what we're doing, that we're not frauds or disgraces to translators everywhere. Not that we really think that with anything else we worked on, just that we're more confident in our work, after thinking about Negima!. That might have a lot to do with comparing the original Negima! translations to our work, but of course we're not trying to condemn anyone, because we've been those translators, too. (Not those adapters, though. We don't like taking that many liberties.)
For example, when we'd read over our scripts to edit them and stuff, we would be laughing at the jokes. We don't take credit for the jokes themselves (except for when we had Chamo saying Negi's schedule was packed tighter than a contortionist in a suitcase--I'll explain that a little more later), because they were Akamatsu-sensei's, but the point of comedy is that it has to make you laugh, and if the translators aren't laughing...something's probably wrong with that. So the fact that we were laughing indicated to us that we translated it right.
As for Chamo's contortionist line...well, in Japanese, the common "figure of speech" type thing to do for emphasis or whatever is to use a sound effect. When we got to that line, we felt like, in English, instead of just trying to come up with one adverb to simulate that sound effect, the "figure of speech" type thing to do would be to use a metaphor. Athena came up with the contortionist bit, and it seemed very much in character for Chamo, so even though we were a little hesitant to take such a big liberty (putting words in a character's mouth), it was too funny and too Chamo (according to our interpretation of the character). So we went with it. We never got our comp copies of that omnibus, though, so we don't know if an editor took it out somewhere down the line.
Also, we feel like we learned a lot from working on Negima!, about all kinds of things. Like gravity, and philosophy about darkness...but also we learned things about how to adapt manga. It made us think about different ways to handle different speech patterns and things like that. A lot of character quirks didn't really make it into the English version--Fumika and Yue both use "desu" in unusual ways, for example, and Chao does have a bit of a Chinese accent--but...we're still working on that. Besides, when there are 21 volumes that came before we started work on it, we have to maintain some consistency.
I wish I could come up with more solid examples of things we learned from Negima!, but the gist of it is that we feel like we've really improved as translators, and like Negima! was really helpful in that process. And that's why it's probably the series we're most proud of our work on.
As for our favorite series...that one's a lot harder to answer.
Today I'm thankful for thought-provoking questions, the opportunity to improve as translators through Negima! and the other various titles we've worked on, having blankets, having time to do some reading if we want, and snuggly warm kitties.