There is no number one in my books.
I've saved my money long enough that pretty much everything I want I can get, though one of my 360's killing itself forced me to trim the fat a little.

I've already got Saints Row 2 and Fable 2.

I'll most definately be grabbing Fallout 3.
A couple of copies of Gears of War 2 so I can enjoy some System Link with my brother.
Same goes for Left4Dead.

Rock Band finally hits Aussie shores in November, but whether I have the funds for it will remain to be seen.
I'll be grabbing it in the not too distant future anyway.