Mystery click puzzle.

Mine appeared under load, every ride. Looked everywhere. Eventually figured out it came from a loose screw in the derailleur hanger. 

Try changing the pedals, I had a similar thing like that. 

Often, things that sounds like bottom bracket creaks are caused by slightly loose chainring bolts (the 5 bolts connecting the chainring to the crank). I wasn't sure from your post whether you've already tried tightening these.

My noise came under load, I changed the BB, swapped pedals.
Eventually, I tightened the rear qr and the noise disappeared. I think there was a slight amount of play in the rear mech which has a group of 5 and 6 individual sprockets.

There is something weird in the design of the bicycle in that all noise seems to be coming from the bottom bracket area, even when the cause may be quite distant.

pedal bearings? swop out pedals - pretty sure your bike shop will lend you some if not got spares as changed the BB with no result - agree on seastpost also saddle clamp(s) grit seems to get in there

...oddest click ever had took me a while to find but fortunately before the end of the ride - metal tab on jacket zip was tip tapping on the top tube

A lot of my creaking comes from the seatpost.  Remove, clean and grease/carbon stuff, and the fastener.  More important in metal on metal interfaces as stops binding.

Check shoes/cleats. I spent ages trying to find my click and it was actually the sole of my shoe which had detached or let water in.

It really could be so many things.  trial and error is likely to be the way forward.  

I had a similar problem and eventually traced it to my rear wheel.  Taking it out and putting it back in solves the problem.  Simply loosening is and doing it up again does nothing.  It has to come out. 

I'm not suggesting this will solve your problem.  It's just to illustrate how, just because the noise comes form one place, it may have absolutly nothing to  do with anything in that area.  Check the headset and bars, too.

Good luck!