Formal:
* אַ גרויסן דאַנק (a groysn dank) - literally "a big thank you"
* גרויסן דאַנק (groysn dank) - a shorter version of the above
* הַאַרבעס! (harbes!) - "Thank you very much!" (literally "Thank you well!")
Informal:
* אַ דאַנק (a dank) - "thank you"
* טאָקע אַ דאַנק (toke a dank) - "thank you very much"
* שׂוֹן מָל (s'hon mol) - "Thank you very much!" (literally "that's good, already!")
You can choose the phrase that best suits the context and your relationship with the person you are thanking.