User Basecamp a lot for client handling. They loves it, because of schedule and reports inbuilt function. I used to allot each team member to each client to make them feel like they are getting followed up everyminute and when they gets email from basecamp that this task is done and this is done they really love it.
And apart from that tools like ai writer, google sheets, dropbox always tops.
Also for invoicing flow i tend to use quickbooks otherwise everything seems to mess up really soon in terms of financials.