I was wondering if anyone might say "any key," or that the key doesn't matter. Ted, I'm totally NOT talking about you in particular here, and this wasn't meant a "trap" in any way, but I would like to comment on that for a sec. I think there's a reluctance to admitting to a favorite key because, for SOME people, it might seem uncool, or like a "negative." After all, if you admit to favoring a key, it might indicate that you're better in that key, which might indicate that you're worse in another key, and some people don't want to admit that they struggle in some keys. Again, NOT pointing fingers here.
I just find that to be interesting, because there are certain undeniable idiosyncrasies with certain keys. As a result, there are certain things that are easier to do in certain keys. That doesn't mean that you CAN'T do them in all keys. It often just means you have to work harder. Of course, this can come down to style. If you play bluesy stuff then you'll likely favor C, F, Bb, and other keys with, among other things, a black note for a flatted third and a white note for a major third, because that accommodates lots of the "sliding" licks in the idiom. Sure, it's our goal to be great in all keys, but it wouldn't be shocking to admit to having a favorite, because it makes certain things that one does easier.
Of course, there are other things that could drive us toward certain keys, such as favoring sharp keys because you either play strings yourself or play with a lot of strings players, or favoring flat keys because you also play brass or play with lots of brass players. Then there are people who have such a strong connection to reading that, even when improvising, they tend to prefer the smallest number of accidentals, because it becomes tedious to think in too many sharps OR flats. And all this is in addition to people who say that certain keys sound different inherently.
Ted, you said "Inasmuch as ever I play within keys," which suggests that you're probably not playing blues all day
However, I'm sure that doesn't mean that you don't have sounds that you tend to prefer as an artist, conscious or otherwise. How interesting that you gravitate towards Db and E, seeing as how different they are! I'd expect your average person who loves Db to shy away from E, and vice versa. I wonder what it is about your musical improvisational preferences that compels you both keys? Interesting!
For me, yes, I play in all keys all the time, but I definitely have my favorite. For years, it's been Db, and as I was saying earlier, it struck me as interesting when I got to thinking about why. There are two reasons:
1. Geography. The geography of the Db scale is so unique that it sometimes feels as if it would be hard to play the wrong note. C major, while an easy key to get started on, actually seems challenging at times because it's an open "plateau" of white notes. It's easier to get "off track" by a note, as opposed to Db, which seems more rigid. For me this seems to extend to the secondary harmony as it applies to Db. All those secondary dominants and tritone subs seem to fit in only one way.
2. The above could apply to several keys, but I think this next reason is what separates Db, for me. When playing diatonic runs in a tonal context (which is not all I do by any stretch, but it has a significant presence in my playing) we often avoid the fourth scale degree. I find myself creating lines that in essence are made of the entire major scale minus the fourth. (Not just running up and down, but in a variety of ways). The two hand positions that make up this scale (thumb on C, then thumb on F) seem to make it particularly easy to play and isolate the fourth (Gb). It just feels comfortable.
Having said all that, I'll definitely favor other keys if I'm playing other styles, such as blues, which I do quite a bit.