There are 10 new mythics and 3 reprinted mythics in Magic 2014. I ranked the new ones by their chance of being good in standard:
Chandra (60%). The point of this would be to +1, survive to untap, and then start getting a free card a turn. It wouldn't go in a 19 mountain aggro deck, and the +0 doesn't interact well with counterspells or Bonfires. Probably a Farseek deck is best; Jund could be adapted to play some cheap blockers -- Elvish Visionary?
Shadowborn Demon (50%). I had this much lower until I read
Owen Turtenwald's M14 piece. "Destroy target creature" less powerful in current standard than it sounds, but Owen points out that sacrificing creatures is pretty easy.
Scourge of Valkas (25%). Obviously this does nothing today, but if, say, two good cheap dragons are printed in the next block, Scourge of Valkas will start looking real.
Garruk, Caller of Beasts (25%). A sideboard card for a hypothetical Bant. I don't know if people will want more than two copies.
Archangel of Thune (10%). You'd need, like, Sol Wardens and Soulmenders to make this work -- creatures that can gain life for zero mana.
Kalonian Hydra (5%). Wolfir Silverheart is underplayed, but this does so much less than Wolfir Silverheart.
Rise of the Dark Realms (1%). I'm not sure how much play 7BB for "Win the game" would see.
Devout Invocation (1%). If you have seven mana and a lot of untapped creatures, there are other cards that just win you the game.
Windreader Sphinx (1%). It's tough to see where this does more than Angel of Serenity / Sphinx's Revelation / Aetherling.
Primeval Bounty (0%). You'd have to be playing a very slow deck with a lot of creatures in it somehow.
The "good mythics" are Chandra and Shadowborn Demon, which will open at (rightfully) high prices.
The mythic I might speculate is Scourge of Valkas -- I'd want to buy low in August and then hope for a Dragon deck in standard by 2014. But the price might not go low enough -- Scourge of Valkas has casual appeal and other people might be making the same play as me.
My plan for now is to sell every mythic rare I open.