Splintergraft Malygos OTK Druid

Generally you want to mulligan for cards that will either ramp you up or keep you alive during the early and mid stages of the game. Wild Growth is pretty much an always keep as well as Twig of the World Tree which help setup the combo. 

Spreading Plague is a great card for keeping aggressive decks at bay while combining it with Branching Paths in the later stages can also create a nasty board of taunt minions. Branching paths allows for you to either draw or armor so you can dig deep for combo cards or keep your health total at a decent level. Harrison Jones is teched in for some weapon removal for nice card draw.