Here toes some but do check if I commit any grammatical mistake.
1.I am expecting your reply. 2.Your reply would be appreciable 3. I still have had no reply despite of writing so many mails. 4. Your answer is expected. 5. By holding conversation, we'll reach nowhere so please reply.
2. Your reply would be appreciated. (This is a nice one.)
3. Despite writing so many letters, I still have had no reply. (Save the big message for last! Make them suffer through writing all those letters first, then tell them how disappointed you are!)