The Community Upgrade Project and RHS are a pretty good place to start. The former has most or all of the units, weapons and terrains from ArmA 1 and 2, including the British units, while the latter has US and Russian forces in rather high quality (especially the Russian stuff). A lot of maps rely on the CUP Terrain pack to work, and I hear you can even use other ArmA 1 and 2 maps with it, so that's another reason to use that. If you want extra weapons, the HLC (or is it NIArms now?) packs are of very high quality.
As far as mod maps go, the aforementioned Isla Duala and Helvantis are quite good for African and European winter scenarios, respectively, especially since these settings aren't really covered by the stock maps. The other maps by the person that made Duala (Panthera, Lingor and Abramia) also look quite good, but I haven't tried them myself yet. Bornholm is another good European map, though it can be a bit heavy.
As Oberon says, ALiVE is really useful for quickly setting up dynamic missions, though it can be difficult to control with a lot of precision. ACE3 is pretty good for introducing a bit more depth to the medical system and various other aspects, and is thankfully not nearly as clunky as ACE2 was.
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