Which is more correct?
Now I am the main stakeholder...
I am now the main stakeholder...
Do the intonations imply different meanings?
Neither is more correct. They mean different things.
You first example can be interpreted like so:
Which means you are about to make a pronouncement about how things will be with you are the main stakeholder.
As a statement by itself it can be used to mean I am now the main stakeholder, but you have to put emphasis on now and make it seem like you are marking the the moment. A similar idea would be to make a future announcement:
Of course, if you put a comma just after now you get a different meaning:
This is using now as an interjection, it doesn't really mean anything with regard to the sentence. The rest of the sentence is just a statement explaining who you are.
Your other example
is a statement explaining that from this moment you are the main stakeholder. This structure can be used in a triumphant way, as an exclamation, for example:
or in a dry factual way:
I agree with the points made by Matt, but wanted to add the following:
If you start the sentence with a designation of time, you emphasize the time.
This first sentence is best used if time is somehow contextually relevant. For example:
In contrast, your other example emphasizes either 'I' or '(the main) stakeholder', depending on context.
The difference is negligible, however.
"Now I am the main stake holder."
A statement by itself it can be used to mean "I am now the main stakeholder." Without the coma, it can be use as a statement regardless of you being the main stock holder now, or in a future tense. Putting emphasis on "now" and it is marking the the moment starting now. The idea is to make a future announcement as well as a present one also.
"Now, I am the main stakeholder!"
With or without an interjection, it does really mean everything with regardless of the time, but is according to the word stated. The statement explains who you are now, and who you are in the very near future. It's a matter of how you mean and feel what you say about something you have stated.