Rav/Raba are the male/female forms of "much". "Toda Raba" and "Toda" are probably all you need in spoken hebrew. "Rav Toda" / "Rav Todot" are archaic/literary/haughty forms of toda raba.

The correct way to say "Rav Toda" is "Rav Todot" That literally means many thanks! Toda Raba also literally means lots of thanks (Rav- many/ a lot) but is used in more casual conversations. Rav todot is much more formal and is not used as much. Therefore, when you want to thank a Hebrew speaker, I would recommend using "Toda raba" just because it sounds more casual in everyday conversations. I hope that helps!