Metallica, simply based on the fact I listen to it more than any of the others' in the poll.
Thrash or not for my money it's probably the best album Metallica ever released, and even though diehard fanatics hate it being called thrash, it did more than any other record of it's time to keep metal alive in the mainstream and for that it rates as one of the most important releases in metal period.
As Rattlehead already knows a little thrash goes a long way for me, I find it really repetitive at times especially Slayer...although I do like a couple of their records.