I'd have to say Project Nevada for New Vegas. IT ADDS VISOR OUTLINES

I've always been fond of Fallout Wanderer's Edition, used in conjuction with Mart's Mutant Mod. It makes the game a lot harder yet so awesome.

Come to think of it I never leave home without Energy Weapons Enhanced or Weapon Mod Kits either...

Fallout 3's Robco Certified and Powered Power Armor. Also, Energy Visual Enhance.

I have many:
Beyond Boulder Dome: This 'mod' felt like a DLC with it's intro, voice acting, a main quest with smaller branches that have to be completed to proceed, and just the feel of it. It was like if Dead Money, Old World Blues, and Lonesome Road were tossed into a blender.
Classic Fallout Weapons: Makes New Vegas really feel like a sequel to the original Fallouts.
A World of Pain: I love exploring and killing things.
NVInteriors: More exploring, killing, and lots of booze!
XRE Vertibird: *Insert maniacle laugh here* Nothing shall stop my quest for booze and murder now that I have a flying death ray!
Any of Impertrator3's weapon mods: These guns are amazing. And slightly overpowered. Not really.
Falcon Pawnch!: *Gives to Veronica* I've just set the scale for which all overpowered ness is measured.
Apocalypse Armory: A 20mm autocannon...a .50 caliber machine gun...a Vindicator minigun...the original Gatling laser...there is a God...and he is FUCKING AWESOME!
Portable Cheat Terminal: It's not cheating because it's a mod.

Wasteland Defence with Enclave Commander.
for me its like "ah sheeeit my 50 or so snipers couldn't handle em WE NEED REINFORCEMENTS *calls in enclave and goes on a rampage*

The Willow Mod. Adds in a Whole new Companion that's fully voiced, properly written, and she has about a dozen side quests to do. She requires you to work on gaining her trust before you even get her perk. She's also romance able.
DEIMOS. Adds in a fully voice, with decent voices quest line that involves time travel to about three days after the bombs fell. It's a bittersweet quest line. One warning though. Do not bring your companions as the creators said that they will permanently die as they kept glitching it out otherwise.
Unlimited Companions perk: Nuff said.
More Perks: Adds in a lot of perks.
Gun Runners Mod: Can't remember the name right now but it makes it so all the unique weapons that were added on are scattered across the MOjave and you have to hunt them down like the rest of the unique weapons.And adds on a mod station to Old Lady Gibsons warehouse the creates the new weapon mods and upgrades your weapons. Also makes the new weapon available to the mooks of the game.
Brotherhood House alliance: Adds in something cut so you don't have to Nuke them to get the House ending.

Wasteland Defense, satchels/tailor maid, enclave commander, project Nevada, Nevada skies, boulder dome, dragonskin tactical armor, EVE, weapon mods expanded, and loads more.

NVEC (New Vegas Enhanced Content)

It adds as many mods as it can find into one easy package.
One of the authors is a debug god.