OK, there's a p1mp thread a few topics down from this one, so I'd like to make an early distinction between these two threads. In your opinion, what are the mods that every FS2 player should really have downloaded and sitting on their computer? The most original, the most useful, or the highest quality of the currently available FS2 mods, not neccesarily your favourite though.
I'd like to begin with the GTF Apollo, the GTF Valkyrie and the GTB Athena from FS1. These are probably used in at least half the campaigns on the net at the moment?, and should definitely be on everyone HD, if only for the memories (IMHO, they should have been in FS2 by default, but that's just me).
Continuing on, I think at least one of the Vasudan installations, either the Cairo or the Karnak to add to the exteme lack of stations in FS2 and possibly Robin Varley's Archeron station, which is very high quality and definitely fits a niche.
Finally on my list of ships are Bobbau's Ancient mods, very high quality and an easy way to add a new species to the game.
The only non ship mod I can think of that's really worth getting is the subspace pallette thing which allows subspace missions for FS2. It's absolutely neccesary.
Now I haven't tried every mod, so I don't know what should and should not be gotten, so any other ideas feel free to add.