i've always been quite found of the mod "The Hive" i believe its by a modder named angelwraith. The Fallout3 Version in my opinion is better than the NV version. So many neat features and references for a house mod.

oh my gosh I think that the most unique mod ive ever used was the borderlands character made right here by dragbody aboslutly loved it but I also remember the clone trooper mod that was amazing too

Busworld - there are a lot of buses and trains in the Capitol Wasteland, people would use them.

Combined with the weather mod that has radioactive rain, and those buses become sanctuaries....

The Auld Grump

Zetman wrote:

I would say Obscurum http://www.nexusmods.com/fallout3/mods/17937/? for Fallout 3.  It really changes the experience of playing Fallout 3.  It turns it into a survival horror game.  

Woah... I'm really gonna have to check that out. Fallout 3 was pretty scary to me when I first started it, but the fear kinda wore off. Looks like it might get scary again?

The rockwell descent for fallout new vegas. That said I got bored of facing waves of deathclaws and other scary monsters. The premise behind it was interesting though.

Have to say True Lean mod. It definitely made sneaking and overall combat better. Also I've recently gotten into the STALKER series again and the wasteland now feels like an extension of the zone! One more unique mod I've used is the Monster Mod.

rePOPUALTED Wasteland is a great fallout NV Mod, the game really seems dull and creepily empty without it.

For me, it's the Advanced Recon Armor mod for Fallout 3. The armors look great and they're very good for stealth characters.

I'd have to say the fallout heroes mod, I'm a big fan of super powers in general, and the mod does a decent if not fantastic job at giving you powers, as well as a story to go with it.

Another would the Titan Fall mod, that one juts screams incredible.

For me, it would he either Zombie Apocalypse, Angel Park or Reactive People.
Zombie Apocolypse is self explanatory, zombies everywhere, no hope in sight, cry lots and hope for the best. 
Angel Park took to world of District 9, Terminator, Alien and countless other movies and turned them into a completely new story and world space. Not only is it the most difficult mod I've played (with difficulty enhancers of course) but it is also a mod I can play for hours. Every building you can explore and each one holds it own secrets. Between new weapons, items and the fact that you can dress up as a T-800 series Terminator, nothing could be more unique. 
Reactive people makes te Wasteland much more immersive and makes it seem so much more realistic. If I'm going to point my fun at you I would hope you do something about it, not just sit there and do nothing. Also when NPCs find dead bodies it hilarious.

Tale of Two Wastelands mod amazed me the most. I never thought it was possible to merge the two games together and now I'm proud that I have found this mod since I no longer have to keep switching between games.

Redesigned 3 is mostly the one I would put on my list, the things it makes is pretty impressive. Every wound or voice that should fit in vanilla does fit. It's amazing.

For me it must be XRE Flying Vertibird, the feeling of superiority and freedom. Flight mechanics and landing are amazing. Compared to XRE Cars, Vertibird doesn't cause my game to freeze or strutter, it just runs much smoother.

nexusmods.com/newvegas/mods/45809/?

Cube Experimental and TSC those two mods were DLC qulity which I really enjoyed you cant really find a lot of quests mods like that, unless you know other ones for fallout 3 then let me know.

    

For me it was bunsakis Operation Mirai mod for fallout 3, ive never had so much fun in combat in fallout before that mod http://www.nexusmods.com/fallout3/mods/14834/?

I would have to say AWOP and Beyond Boulder Dome. Story and gameplay drags me in for the long run and these two sure did that!

The Destruction Overhaul is very interesting, you can basically destroy unimportant objects around the environment. As of now the mod is pretty barebones but updates are coming in quick so give it a try.

The first thing that came to my mind is AlexScorpion's AK-103. I remember a Desert Eagle mod similar to it but I have not looked for it. What makes it unique is you can attach and detach more than three mods for the gun, there are five as appose to the normal 3.

Spoiler:

  Lol I'm sure someone will notice the amount of caps and weight, we all like to cheat in some way, just saying this so nobody has to say anything about me cheating in the game.

I'm going to have to hand it to Tale of Two Wastelands as well. I just enjoy the concept of starting out in the DC Wasteland and I love how it takes you to the Mojave in an immersive way.

X-1 Tales of Wonder - Radio Station for fallout 3

@ DB8 lol mate I'm the same fallout games turn me into a hoarder I need an insane carry weight also so I can pick up all that wonderful clutter lying around someone has to clean up the wasteland after all