Worded like this makes it sound as if you are waiting as soon as possible (for the reply).
I look forward to hearing from you as soon as possible.
Please reply as soon as possible.
We don't usually say "I'm waiting for..." when asking people to reply. It sounds a little impatient, as if you are sitting at your desk, tapping your fingers, waiting expectantly for the email/call to come! :-D
However, we do sometimes say "I await your urgent reply". This expresses the urgency of it!