I have an uncle who is giving me a few million from Germany he lives there and has already paid taxes on the money in Germany. Do I have to pay taxes on it again? I will just be investing most of it.

I'm in California but I transacted some business completely out of the country but am getting paid here. It is several million dollars. Is there any way to pay less than 35% and 10% state taxes?

If I got paid in Mexico next time and paid Mexico tax (I'm dual citizen) and then transferred it here would I still pay when it came to the US?

This is your U.S. source income, so you must pay tax on it.

Even if you were paid in Mexico, then still it is U.S. source income and you will pay normal tax on it. You will not get any credit for taxes paid in Mexico.
I do not think that this income should be taxable in Mexico.

If your uncle is not a U.S. person (U.S. citizen or resident), then there is no gift tax involved.
In the U.S. receiver of the gift does not pay any tax; only the donor pays the gift tax. If your uncle is a foreign person, then he is not subject to U.S. taxes.

You will have to file form 3520: Notice of Large Gifts Received from Foreign Persons. The form is due of the date of your return including extensions.The form is filed at Internal Revenue Service, P.O. Box 409101, Ogden, UT 84409

My goodness, MILLIONS!

My advice to you dodger, is to find the best tax professional your millions can now afford and have s/he help you with your tax planning and preparation. I mean really, what are you doing here?