Full XT with is the best value.

The following will provide the best value upgrades, in this order:

Shifters
Shadow Plus (XT or XTR) rear derailleur (especially for 29ers or dual suspension bikes)
chain (bit of weight, only ~$15-20 more, but lasts a little longer, so it's a wash in terms of price)

I wouldn't bother with cranks, as weight difference is so small and shifting is excellent with regardless with 2*10. XTR chainrings are nearly double the price of XT.

I wouldn't bother with the cassette as the titanium cogs wear quite quickly on xtr cassettes.

Front derailleur will make no difference.