Although, I don't have a favorite splinter, I will write about one that I find truly special. I'm talking about Death Splinter.
This is a splinter that is dear to me because the first reward card I got was a Phantasm. I always felt attracted to that kind of cards perhaps for its depiction of the creatures and their powers. From a practical point of view, this splinter is full with cards that give you the flexibility to use them on high mana as well as low mana fights. For example, you can use a devastating tank as Undead Rexx or a steady and effective Haunted Spirit with success.
Their basic summoners came with useful debuffs that can diminished melee or range attack, depending on the summoner. One weakness of this splinter is that almost all if not all my battle draws have been using this Splinter. Alas, use it with caution: too much usage of this splinter is addictive and could end up stealing your soul. Players beware!