I say bring back the mammoth because there is a lot of habitat that could use it. Think how much people would pay to see woolly mammoths in their native habitat.
Somehow, I believe the law of unintended consequences would happen, reintroducing an extinct species will impact the natural order of things and no one has thought about the negative side of that.
You would have to bring back more than one Woolly Mammoth because depending on their lifespan they will become extinct again as soon as the first one dies off.
If you are going to continue the species you would have breed them to keep it going.