What should I use to patch this gap between brick siding and top of foundation
Take and angle grinder and cut out the metal as much as you can.   The part sticking out isn't doing anything.   
Use an epoxy concrete kit to fill in the gap.
Skim on a layer of concrete patch - to even out the corner and cover epoxy.   


I would not just fill this with a concrete patch.   It will even pop out of that gap.  The skim layer may eventually fail too but it will have no bearing on water getting in and can be repaired in minutes.