What kind of belly bar is best to use?

Considering I have been in the business for over 15 years I have yet to run across a "Belly Bar" so I will hazard a guess that you mean a navel banana bell or a bent barbell or a navel gem. Having never seen anyone pierce a "belly" I am again guessing you mean a Navel piercing, "which is the upper tissue fold as one is standing directly above the umbilical cord "knot**" in the navel area (**commonly referred to as the belly button). The jewellery of choice can be 316 LVM Surgical stainless steel, Implant grade composition "D"steel, Titanium grade 23 or higher, PTFE, Bioplast or Niobium. These can be bent or curved barbells, rings captive bead or fixed bead, circular barbells. Selection of the best style of jewellery is generally done in concert with the body piercer and the client at the time of the piercing.