The best chocolate to use is high quality chocolate baking bars. Semisweet, bittersweet, milk, and dark chocolate are all perfect- it just depends on your flavor preference. The key is to use chocolate baking bars that are sold in the baking aisle. I recommend brands such as Baker’s, Ghirardelli, Nestle, or Lindt. Avoid using chocolate chips, as they won't melt as well and you won't get much "lava" from them.