I don't think there is a best race for each class. There are some obvious synergies (Halfling Priest, Gnome Wizard), some combinations that might not make a lot of sense (like an Orc Wizard), but I think there are a lot of equally valid choices that mostly come down to playstyle, the other preparations you're taking, the particular dungeon, etc.
A nice safe starting point is to use Gnome if you are primarily magic based, and Halfling if you are primarily attack based. Extra potions help you out of a number of situations.
Then you can expand from there into trying Human, Dwarf, or Orc for attack based, and Elf for magic based. Their bonuses are a bit more subtle than extra potions, but they can be more powerful since they have an impact throughout the level.
Goblin can work well for any class, just make sure that you are focusing on using your conversions when you most need them (like to level up to fight the monsters you can see, or to get a mid-battle levelup).
Eventually you'll start finding that even weird combinations can work quite well. Elf can help a fighting class get to the point where you can cast 2 BURNDARAYZ in a single battle, which can be a big difference. Or it can help get you some extra magic points to sacrifice to Taurog. Human can help offset the melee damage penalty for Wizard. Maybe the dungeon has a boss that does a bunch of damage, so you choose Dwarf to help get to the point where you can survive a hit. So the most important thing is to try lots of different options, and keep track of what works well for you and your playstyle, then go back to that when you are struggling with a particular dungeon.