My feelings on the exact moment Michonne knew Rick was it for her.
"I'm still with you."
The enormity of those words hit her like a ton of bricks. Did she really just say that? Knowing that she fought so hard to push Rick into seeing that they needed this place. They needed a safe haven. A secure place away from the horrors of being out there. Out in the open spaces where walkers and bad men alike were their number one enemies. Or maybe that wasn't the biggest enemy. Hunger and no shelter was perhaps on the top of that list.
And yet, even though Rick put all their lives in jeopardy by going on a rampage. His actions threatening to get himself and perhaps all of them kicked out, she knew she would follow him to hell and back. Even if Deanna just decided to kick Rick alone out, she might as well be kicking her out as well. Their fates were intertwined.
It was in that moment she knew she loved Rick Grimes. When her safety no longer mattered to her, but just being with him is all that counted. She knew.