For me, only three absolute ‘must have’ add-ons:
(1) Bijan Seasons mod
(2) We love VFR
(3) Global AI Ship Traffic
the first two are absolutely transformational to the sim. Now I know that the Seasons mod is payware and so not on flightsim.to, but it improves the default world so much (in my meagre opinion) that if by any slight chance you haven’t heard of it yet, you really need to check it out.
The value of the last obviously depends on where you fly; but I’d echo what was said above, the Great Lakes addition is a fantastic and I’m thoroughly enjoying it cruising along the shores of lake Michigan in the Junkers at 2000agl 
Now of course you can also see my preferred type of flying from this list…