well, right now the one I'd say is a must is the HiFi Novus Aevum sound mod...some of the best weapon/explosion sounds I've heard. The vehicle sounds are kinda weak; but it's modular so you can remove the vehicles sound .pbo and use another sound mod.
Other "must have's":
Enhanced Combat Environment - Adds a lot of ambiance to ArmA...birds chirping, wolves howling, insects buzzing, etc. Also has a really nice fog effect. Also changes AI behavior so they run from grenades, use grenades more, and will re-man MG's or towers if the operator gets killed.
TrueMods - changes the game GUI to something a lot more streamlined and sleek...no giant reticules or menus. Fully customizable if you don't like a particular feature.
Z_guba's model fix - If you have a GeForce gfx card; you might get the Javelin bug (you get crazy texture lines shooting out from anyone who is carrying the launcher). This fixes it
ArmA Effects or JTD Smoke effects - Changes the look (not sound) of explosions, particles, and smoke. I'm a little more partial to ArmAEffects; but both are really good.
Sixth Sense tracers - Changes the default tracers to something a lot better. They even cast light on the ground/objects at night and looks really good (you can see them in the ArmA MM screenshot threads).
Proper plants Autumn) - also available with Kegetys lowplants, and/or "stock" colors
"1. Przemek kondor beautiful Autumn plants
2. AI view is blocked by the tree/bush object (better)
Check Durg's (AI View) Vegetation Fix for details and screens.
3. Ability to hide in bushes
(by removing all geometry points)
4. To run over trees without resistance with the heavy tanks
(by setting overall geometry mass to 200)
(unfortunately no 'groan' sound event possible like in OFP)
That makes mowing down loads of trees again.
Speed reduction still applied for smaller vehicles.
ACE Island Pack will feature separate tree models for the forest
that will slow down even heavy armor.
5. Ugly white / bright shader (OLD / GONE) removed by kegyets lowplants software"
I also have:
Modul_sky replacement - changes the vanilla sky to something more photo-realistic.
Italian soldiers replacement - The best looking camo pattern that I've found for ArmA's scenery
Proper weaponcollision - changes the collision on your rifles so you can hug walls and go thru buildings without getting stuck (makes CQC a lot better)
Andy_Mags - changes the default 'bag' model when you drop ammo with models that actually represent the ammo dropped (i.e. 30-round AK mag looks like the real thing)
Sky and Weapon replacements - I don't use the sky cause I already use modul's; but this replaces every weapon with better looking models from various weapon packs. Also adds animations so you can see the bolt move when firing, and the covers on LMG flip up when reloading.
ugghhh...i think that's most of them lol. I think you can find all of these at
www.armaholic.com or on the BIS forums. Really it's just personal preference; so you'll have to do some testing to see if they work for you. There's a few campaigns and singleplayer missions that are really good; but most require a ton of addons to DL and install and I don't feel like doing that. But if you're up to it, they're all over at armaholic also
) and I have sets for different time periods from 1985 to 2008, so really anything's possible