Theyre meant for playing right out of the box, making them ideal for newcomers and for learning a sets new mechanics
For seven, it clears every nonland permanent your opponents control
Defiler of Flesh converts a single black pip on each permanent you cast into Phyrexian mana, which sounds great, but the cast trigger is pretty tame compared to something like Defiler of Vigor or Defiler of Dreams
The easy picks are going to be The Gitrog Monster, Crucible of Worlds, and Ramunap Excavator
The one Karakas can be a third Plains if your local metagame has a lot of Wasteland and not a lot of Reanimator, Show and Tell, and/or Control, but I have found the exposure to Wasteland to be worth it to include Karakas in the maindeck
Instead of boosting power and toughness as a static ability, this Elf Warrior creature features a mana sink (and what a sink it is, with an activation cost of ) that boosts the power and toughness of all of your Elves by two while tacking on deathtouch