On my Ultimate Hacking Keyboard I have a mapping of (left)space+q to produce Esc which for this combo I hit comfortably with my left hand thumb + ring finger.

On other keyboards I find fishing for the Esc key with the left ring or sometimes even middle finger good enough since it's so prominently singled out on the top left corner on all the keyboards I had yet to use, but it breaks the flow a bit more.

It probably helps that I'm not typing in proper form for the 10-finger system but only some hodgepodge approximation of it...