Giratina is my favorite in design and lore, but having power over antimatter doesn't seem useful for anything but destruction.
Messing with time with Dialga sounds temping but I think the possibilities of branching timelines caused by time fuckery would just consume me until I became a total perfectionist.
I think space related powers are easiest to apply to my existing life style. Get to places in an instant, transport whatever I need at whim etc. So Palkia it is.
