They're both grammatically correct, but neither one is very descriptive. Just say what you're doing: I'm writing an email. I'm organizing my files. I'm contacting potential clients.

These are basically the same, but the way your voice sounds when you say them can convey some extra meaning. For example: 1. "I'm working on **THIS** task." = You are showing which task you are working on 2. "I'm working on this task." = You are telling someone what you are doing in a general sort of way. "This is the task I'm working on." = The feeling of this is more like #1. Good luck!

They are equally good. Your intonation is more important than the words that you choose. Which word(s) do you choose to emphasize? "This?", "I?", "task?", "working?", "the?" That choice makes all the difference. In comparison to that choice, the question of which variant you use matters very little.