I
had to pick
pies. I just can't make a qualified judgement. It's a whole package of awesome stacked on awesome, with layers of win in the middle. I love the universe, I love how the roleplaying is there for entire civilizations without being too constricting, I love the setting, I love the added features, I love the mechanics.
Of course, it's not a commercial-grade project, it's a mod and there are small annoyances, but overall, this is by far and wide the best mod, all categories, that I've ever, ever played (and I'm extremely, extremely picky about my mods). I wish I could send Kael my money, because this is greater than most expansions for most games.
Take out any one part - the features, the universe, the flavour, the diversity, or the high standards (models are rarely, if ever, re-used for multiple units- just as an example), and it could just as equally fall flat.
I refuse to single out any one part of Fall from Heaven 2. It'd be like
"What's your favourite part of your car? The tires, the steering wheel, the engine or the brakes?".
It's just a too great of a mod that ends up so much greater than the sum of all it's parts.
The universe, easily. You'll notice I spend more time in the Lore forum then the main one. The FfH universe is about as well developed as many famous IPs (i.e. Star Wars, Star Trek, Lord of the Rings...) and manages to be believably dark without going overboard and into Grimdark territory (where the level of darkness wraps around into satirically overblown).
It's not Grimdark until you've got Inquisition, genocide and rape on a daily basis.
... In a cellar. Without lights. Underwater. On acid.