For me its the left space bar, with the right space being space. I also have them mapped that if I hold them down they active different layers. I like the two most used keys on my keyboard to be for my thumbs as I like using both my thumbs. A single space bar, particularly a huge one like a 10u is more aesthetic but its slower for me to use, really depends on what your typing style is.

However you can make any key anything you like, even macros, so you could put it anywhere you like. I find using the same basic map on all my keyboards is best as I am not hunting around working out where things are.