I've been playing with color contrast for a bit, but I'm not sure if it's just my eyes/monitor that is biased:

Which one of these is easiest/hardest to read for you? (most/least contrast)

According to APCA (https://git.apcacontrast.com) all five have the same Lc of 30.

For me 1 & 2 (light on dark) are much easier to read than 4 & 5 (dark on light)

APCA

This is the APCA project repo on GitHub.Documentation linked below.

APCA
@Typeface same for me. I think that maybe it has to do with thin dark glyphs being “engulfed” by the light of the surrounding pixels, quite the opposite to print glyphs, where a white text gets engulfed by the surrounding dark ink.
You could make a test with very bold fonts just to check if the weight of the font is the issue
@fdrc_ff good point, that could skew this particular test. I'll have to check if the APCA calculation embeds this. If it doesn't then dark-on-light should use a higher Lc value than light-on-dark, to get the same perceived contrast (for smaller fonts).
@Typeface to me the hardest are 3 and 4.
@Typeface 2 best, 5 worst - iPad mini with paperlike matte screen
@Typeface best to worst 1,3,2,4,5 iPhone 15 Pro
@Typeface 2 and 3 are the easiest for me.
@Typeface They are more or less the same to me as soon as I look at them separately and not side by side. Seems that there is some simultaneous contrast at work here.

@larswinter surrounding colors might indeed influence this. I use dark mode everywhere, so the lightest tile is much brighter than anything else on screen.

But 5 still appears to have considerably less contrast for me even if I view it separately.

@Typeface On my iPhone in medium ambiant light and 50% brightness they are in order from best to worst.
@Typeface in order on my iPhone, most to least: 1,3,2,4,5