There are quite a few object libraries for various sceneries which will be 'must have' files for many depending on what airports, ect. they have downloaded.
I would second puffinflight's welovevfr parts 1, 3 & 3 along with mamu's powerlines and marinas mods
Of course, FSLTL will be a must for anyone who likes their airports populated with real airlines rather than the rather awful default liveries.