I liked Nightfall the best because I really hate PvP, also, I liked the Primeval Elite armor for my Warrior. Factions is my least favorite campaign, because as you said, it so PvP based. Eye of the North was my favorite, then Nightfall, then Prophecies.
Oh no, I've completed all the campaigns, just not on one character all at one go.

I (rage)quit playing Guildwars when my level 17 Survivor had his first death. I've tried to start playing again, but it's just so dull and boring now that I've done it all (and it being a 6 year old game).

Beating Maximo Ghost to Glory without using a single death coin. By the time I beat the game I had like 15 death coins.

I always feel quite accomplished after winning a battle against ridiculous odds in shogun 2.

Played the first Tony Hawk Pro Skater all the way through the night->day I bought it (took me close to 24 hours)

Beating Wolfentstein 3d was pretty awesome, too.

Getting the dolls in Exhumed, on the Saturn IIRC.

Finding the six mini-game in San Andreas. Hot Coffee!!!

The most satisfying feeling in a videogame for me is taking down a chopper in Battlefield 3 multiplayer manually with an RPG. Also coming in first in the 24 Hours Of Le Mans in GRID.

Beating up on Sam in Evil Dead Regeneration made feel like a big man .
Defeating Ghaleon in Silver Star Story on the PS1 made me feel good.
Defeating the Cyloid Emperor in Duke Nukem: Total Meltdown.

Getting all the achievements in Borderlands.

Honestly, Moxxi's underdome was almost unbearable.

Finishing Deus Ex. I've done it dozens of times, it's not even that hard, but every time it leaves me with an immense feeling of satisfaction. Every damned time.

Finishing a game. It happens so infrequently for me it's always an achievement when I do.

Probably the things I did in the multiplayer world (even if I always did it offline) in Dungeon Siege. I navigated this super-maze called the Pit of Despair not once but twice. I found the Lost Pyramids in the Endless Dunes (called that for good reason) not once but twice. And I created a character from scratch and used him to overcome the Trial of Gallus (probably the hardest thing you can accomplish in the game) and gained possibly the best weapon in the game.

Edit: And I did all this without the aid of cheats.

all of the boss monsters in FFXi that I soloed with my monk(before level cap raise!) that no-one believed was even remotely possible. Then doing it again when people didn't believe me.

Beating Mirror's Edge on both PC and Xbox without using any guns. Granted, I love that game to death, but still.

Beating Baldur's Gate 2 + expansion (including official difficulty-boosting patches) with an all-bard party.

Beating I Wanna Be The Guy.

Getting the Mega Man Anniversary Collection for the PS2, and finding that the years away from oldschool platformers had not left me unable to play them.

Reaching level 99 in Dragon Quest 8.

getting to the last mission of Reach on Legendary. However, all my friends bailed when we were about halfway through the last platform place thingy.

None.
I don't see finishing a video game as an accomplishment.

do I have found that it is ultimately the hard games that give the most accomplishment.
I recently played through Star Wars Jedi Knights II: Jedi Outcast
that game is cheap, it is frustrating, it is hard, and it is unforgiving.

but by Jolly-God I beat that SOB, I freakin' beat, and I felt more accomplishment beating that game from my childhood than I did from most modern-day games.

Donkey Kong Country, which I've also been playing through gradually is also surprisingly tough, but man I really do feel relief and accomplishment when I beat a level (most recently Tree-Top Rock, damn that level was hard >.<)

as for specific events? I don't know, Taking on a Rakk Hive with my friends in Borderlands maybe? Beating Too Human with a friend of mine, or playing through the entirety of Saint's Row 2 with that same friend (We 100% that whole game, took half my sophomore year of high school to do it too, we actually finished it the day 10th grade ended )

but most of this was at least two years ago, I can't think of anything significant recently, though I don't play games as much lately.

Actually finishing Grand Theft Auto IV. Not that it was a hard game, but I haven't beaten a game that long since the last generation.

When I 100% fnv for achievements and when I beat gears 3 on insanity

While playing X3: Terran conflict with the IMproved Races Mod, I repelled a Xenon invasion of Zyarth's Dominion, and then fought the Xenon back through the jump gate to Xenon Sector 598, where I conquered the sector and took it as my own. It ended up being a real pain in the ass to maintain control of the sector, but taking it in the first place felt awesome.

Hmmm.... would either be getting all 108 stars in Suikoden I-V or across-the-map Tomahawk/Throwing Knife kills in CoD. I dunno, I just feel so awesome when I get them.

Completing Might & Magic 7 with lvl1 chrs (never training to level up) so you could finish the game faster (speed game). Not because I was ever the best/fastest at doing it...but because of all the planning/forethought it took to accomplish it. Using the skill point pedestals and horseshoes to get enough points to raise some skills to where you needed, knowing travel schedules & rushing to do them in a good sequence to cut down days passed, getting through certain areas/fights at that clvl and so on.

There was also one time where I finished the last "peaceful" mission of Caesar3 and was really proud of how I'd done/arranged things to do it - carefully planned Palace blocks/roads, perfect scores in everything, etc. Stuff like that.

Altho, finishing main plot/all levels of any game is always a nice thing, too...since I only do that about 40% of the time.

When you cut to the animation of you jumping on the dragon in Skyrim and you start stabbing it through its skull.

Being regarded as one of the best players in an online gaming community always beats any single-player accomplishments for me.