I always feel uneasy about letter endings. I sometimes use (1) Yours sincerely, (2) Yours faithfully, and (3) Yours &c. But none of these seem an appropriate response to a correspondent's Best wishes. (1) and (2) would be too formal a response, and in (3) there is, I suppose, some jocularity and familiarity, which may not always be appropriate. My unsatisfactory solution is to drop the valediction entirely when I have received Best wishes. In a way, I feel Best wishes is too informal---and also inappropriate because it assumes I /need/ my correspondent's best wishes, it is almost like saying 'good luck'---, and that in using it my correspondent has forfeited the right to receive any valediction in return. Or have I missed something, is there an appropriate response to Best wishes, if my sentiments are granted?
To put the question another way, what is the most formal response to Best wishes that will not be awkwardly formal in contrast?