More/Most important is often used to introduce an assertion. It's like the shortened down version of "What's more important" or "What's most important". To make this point more plain, just add the accompanying words "than that" or "of all".
"More importantly than that" - Not good
"More important than that" - The good grammar choice.
"Most importantly of all" - Not good
"Most important of all" - The good grammar choice.
- I like camping. More important is that I get to hang out with my friends.
- What's more important in life than happiness?
- I have a good job. What's more important is that I like it, even on the stressful days.
- I like big books and I cannot lie. Most important is "A Fine Balance", by Rohinton Mistry.
- The Princess Bride is the most important movie in my life.
- Foreign policy ranked as the second most important election issue this year.
The "important" rule: Stay away from the "LY" when using modifiers like most and more.
I've never seen a valid use of more or most importantly.