I'm curious to know if it is incorrect to say, "I'll get back to you at MY soonest opportunity", or should I say, "I'll get back to you at THE soonest opportunity"?
I agree with Ash--use "earliest", as in "earliest opportunity". Back in the days when telephones were still comparatively rare, people sent telegrams, not e-mails. Telegraph companies (e.g., Western Union) charged by the letter, so people would write "soonest" (7 letters) rather than "at the earliest opportunity" (24 letters and 3 spaces) to save money. Hence the term "telegraphese". The ultimate in telegraphese is this: It's said that Victor Hugo, wanting to know how his new novel was selling, cabled his publisher the single character "?". The book was a best-seller, so publisher cabled Hugo back the single character "!".