I went for Chaos lord, he packs enough surprises to keep you on your guard untill he's dead, but is easy enough to beet most of the time.
Other contenders were Graviton and Rainbow for simmilar reasons, but those 2 have a terrible tendancy to kill me over 70% of the time.
Octane can be a fun fight if I get alot of SoG out early, otherwise it's usually over before it's really a battle.
If I were allowed to pick my deck before fighting the FG (aka oracle), Dark Matter and Divine Glory would definatly be my top 2.