"Now I am" vs. "I am now"

Which is more correct?

Now I am the main stakeholder...

or

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:

Now that I am the main stakeholder ...

Which means you are about to make a pronouncement about how things will be with you are the main stakeholder.

E.g.

Now I am the main stake holder, all developers will have their wages halved.

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:

From 6pm today I will be the main stake holder.

Of course, if you put a comma just after now you get a different meaning:

Now, I am the main stakeholder.

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

I am now the main stakeholder.

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:

Jensen Button is now the winner!

or in a dry factual way:

I am now the answerer.

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.

Now I am the main stakeholder...

This first sentence is best used if time is somehow contextually relevant. For example:

I used to work as a bell boy here. Now I am the main stakeholder.

In contrast, your other example emphasizes either 'I' or '(the main) stakeholder', depending on context.

They thought I'd always stay a bell boy, but I am now the main stakeholder.

The difference is negligible, however.