The Standard Polymorph deck relied on using creature tokens created through Khalni Garden or Awakening Zone to reliably bring out a single large creature. Early versions used Darksteel Colossus, Progenitus or Iona, Shield of Emeria while Rise of the Eldrazi variants often included Emrakul, the Aeons Torn. The deck is forced to adopt a controlling strategy, both to buy the time necessary to find a Polymorph and produce a token, and because countermagic is needed to prevent the opponent from destroying the token in response to the Polymorph.
Creatures used as Polymorph targets share the common features of being difficult to remove (due to indestructibility, protection or Iona's ability which can be used to deny the opponent the ability to play answers) and having high power and evasive abilities (flying, protection or trample) to end the game quickly.
It should be noted that since Emrakul is not cast, a player using Polymorph to get Emrakul on the field will not gain an extra turn.