[LeapingLennie]

I use C-Space. Easy to type and doesn’t interfere with any readline or vim keybindings.

[psynapse]

Same here. I also use space as my Vim leader, so it feels natural.

[biggerfisch]

Same here to both. Only real conflict is with dunst [0] which also likes to use Ctrl-Space by default.

[0] https://github.com/dunst-project/dunst