Those giant six foot high turtles.
Cite? 
The problem is lack of habitat. You need not only a time machine but the ability to clean up their rivers and keep them clean.
The other person who suggested bringing back passenger pigeons has already pointed out that bad things would happen as a result.
Tasmanian aborigines. On the whole I believe the British Empire did more harm than good, but we sure as shooting dropped the ball on that occasion.
ALBC and It’s british cousin
All of them.
Plesiosaurs, definitely. I think they’d be so cool.
Female engineers.
Did you mean to say that the Empire did more GOOD than HARM?
Hallucigenia
Or six-foot dragonflies.
How about the sabretooth tiger? No, better: the elephant bird.
Well, with a big enough time machine, we could bring back the habitat as well! 
(Finding a place to put it is left as an exercise for the reader.)
Didn’t you see 10,000 B.C.?
No> Good choice.
Yes. :smack:
Dude! You’re supposed to BLAME the DEMOCRATS when you’re caught in a mistake like that?
Have I taught you nothing?
Back to the thread topic: I’d bring back the smilodons. For entirely un-evil reasons, I promise.
Libris Alexandria 
If dragons ever existed, then that’s an easy pick. If not, I’ll say brontosaurus.
Only if you put them in Loch Ness.
The quagga, although it was technically a subspecies of zebra. It’s got a great name for Scrabble and other word games, and it would be nice to have photographs of a variety of examples, as opposed to the one mare that was committed to film before the subspecies officially went extinct in 1883.
This dream is one of the more realistic ones mentioned in this thread – not only are there over twenty preserved specimens (and thus DNA) in museums around the world, but “back breeding” has produced animals quite similar to historic quaggas.
Those giant tortoise that once existed on, I think, Easter Island
There’s gotta be some giant bird with a big dangerous beak that’s be fun to watch roam the plains in great flocks.
Did the Dodo have a big cousin?