Whatever key it's written in. I think sharps/flats sound better? I do about 6 songs in C, because it's not a good singing range for me.

I adjust the key of any song to suit my vocal range and then play in that key, whatever it is.

I can usually play any song I know in whatever key is required should someone else be singing.

Whilst I've never really analysed this scientifically, I don't remember songs as discrete chord sequences in a specific key, I seem to remember them as relative intervals from a start point.

I have no idea if this is strange although a guitarist friend considers this as some form of magic!

I don't sing, so that's not a consideration for me personally.

When I'm learning a new song, I'm usually very excited to start playing it, and I'm too lazy to transpose the dots. So I end up learning it in whatever key it's written. That might change if I can successfully make the transition to an electronic score display program. But I have so much paper music!

I agree with Bill Lewis- I'm sure I could recognize and memorize patterns better if I transposed everything to C. In the long run that would probably allow me to play more songs from memory, leading to fewer mistakes.

Again, the key enabler for learning new music in a specific key is transposable electronic score sheets. I even bought myself an iPad for this purpose. But so far all I've used it for is to display scanned images of paper music sheets that I already had :-(

Getting a little off-topic, but one thing holding me back... With an electronic score display, how do you add annotations to the score??

"Do you always play in the key of C?"

Yes.

When I started out, in another century, I played mostly in C. Then I learned G, then F, then D, then A, then Bb, then E, then Eb. I could get by in the other keys, but not as easily because I just don't play them much.
I think in chord intervals, like I, IV, V7. Instead of a G aug, if I'm in C, I think of 5 aug.
I sometimes transpose if a particular song is easier in a different key and I need to sing it in a different key, or if a certain "lick" is easier in a certain key.
For example Floyd Kramer type piano riffs that use his signature grace note thing are MUCH easier in white keys, C,F,G.

For keyboard, probably , yes, most of my keyboard music Is in key of C.
For piano whatever key it is written in.