Things to consider for each of them:
Emma - how much do you love the character? If you love her or you don't have much decent blast or elemental types, get her.
Agent Venom - do you have him at T2 already? Again, how much do you love the character? There are other better combat heroes to get.
Medusa - do you have the Satana uni? If yes, no need to get this.
Hela - CTP is always cool. Get this if you don't have any other good female villain (Proxima, Enchantress, Titania)
Ghost - do you have her at T2? Do you have the Vulture uni? If you answer yes and no respectively, get this.
Ant-man - NO
Magneto - are you having a rough time at Blast Villain ABX? If yes, this would double your score with ease.