I am really confused about which manufacturer has the best composite material to use for a rather large deck. I narrowed it down to Timbertech, but after reading so many posts about Timbertech and other composite boards (mold, mildew, etc) I don't know which is best. Any opinions?

As of lately i would venture to say Timbertech, Millenium and Geodeckin that order and by my own personal opinion. But, there are many more companys popping up outta the woodwork every year. And another thing,when they say maintenance free, its a pretty bold statement and far from the truth. It will still need cleaning evey so often.Some even fade in direct sunlight and it cannot be helped no matter what a salesperson says. Worse case scenario they can get scratched and look mighty ugly, thats when you might have to replace that certain piece and after doing so, you will find out how much it has faded.
But, whatever you product you decide upon, you get what you pay for.
To date, i havnt seen any mold or mildew in any of the decks i have built with Timbertech, and millenium is pretty new but,it sure looks like a top notched product to me. Geodeck, is another very fine product too, and i have yet to see any blemishing in it.
Good luck, in making your decision and remember, you get what you pay for if you compromise with a cheaper product.