You get to go back in time and bring enough of a single species forward in time to save it from extinction. What do you choose?
Trilobites. Just cause.
It’d probably turn out that they’re filthy little creatures, though, the cockroaches of the sea. And maybe poisonous to boot (which is how they managed to live so long). That’s be the situation with MY luck.
Pie.
T. Rexes. Because when I reveal my secret plans to take over the world, then I… Oh, crap. Forget I said that.
Or dodos.
Tasmanian tiger. They’re so cool looking.
Megatherium. Giant, armored ground sloths are cool. And, of course, quite slow… so when bringing them back all goes horribly wrong I’ll be able to outrun them in the inevitable, low-budget SciFi movie.
The passenger pigeon. I didn’t even have to think twice.
Bad things would happen as a result, though, so I could only do it as Fabulous Creature.
Sivatheriummight be a fun choice. One of the bridge animals between Giraffes and Okapi.
No wait, the Steller’s Sea Cow. Gentle Giants of the Sea. Discovered in 1741 and extinct by 1768. 
I came in here to say that. Just because . . . passenger pigeons; what we did to them.
Whigs. We really need a third option.
Neanderthals. For the cage fights.
The Dodo. Just to screw with the all of the extinction sayings that have to do with them.
Weren’t they pretty horrible pests, though, in terms of destroying crops and such? If you can only revive one extinct animal, maybe it should be one that would do less damage.
The Whooly Mamoth.
I think they’d be totally cool to have around.
That and it might take 10,000 to make them viable again.
The Megatherium would be cool.
My last name is Wooley and Mammoth is my totem creature.
You know which way I’m going with this. If I hit the lottery I’m backing those cloning guys all the way.
Velociraptors, just to screw with XKCD.
Those poor Yangtze river dolphins.
Homo floriensis - if they were real. Otherwise count me for woolly mammoths.