Nothing? Not a competitive type, I guess. I like some combat challenge but huge boss battles or tough encounters are mostly just annoying.

Same here. I was quite proud when I finished it on the 'Realistic' difficulty setting with barely firing a shot. Think I only killed about 5 NPCs throughout, and that was only because the game forced me to, or when I really felt they deserved it.

With the help of a friend I also finished the whole Baldur's Gate series with the same character in one go. We took it in shifts to play six hours each, and the game wasn't shut down at all. It took about 8 days solid and now I never want to play it again.

Challenging myself to new hair pulling levels of difficulty. Also finishing a good RPG always feels good.

Edit
Pub stomping some newbs in TF2 as a heavy. Hahahah, they knew me by name and I only very rarely appear to play there. I can also be simply refereed to as http://www.youtube.com/watch?v=o6qAIaqK3_Qcoming to screw up their day.

Managing to finish all the Skyrim questlines, with a totally bugged ("test"-)character. Why, not rage-quitting was an accomplishment of its own.

Halo reach legendary solo. That or "foxiest of hounds" in DEHR.
Just remembered getting 1000/1000 in Dark souls. That wins out easily, I [censored] hated the four kings on NG+.

The hardest thing I tried for was trying to get all the chivements in mirrors edge. I failed hard, those time trials drove me insane.

Being lucky enough for a Windforce to drop.

In Dark Souls, I was occupying myself by invading players in the Duke's Archives. In one instance, there was no one waiting by the usual PvP spot, so I ran around the castle searching for my quarry. Seeing all the dead corpses littering the place made it clear that the host was not inside, but had to have gone outdoors towards the Crystal Caverns. I went there and to my surprise, saw a whole bunch of dead Crystal Golems there too...he wasn't heading towards the caverns, he was in them! I didn't have much time before he'd reach Seath the Scaleless. So I rushed through the caverns, practically sprinting across the invisible bridges until I finally caught up to him at the area with the Man-Eating Shells, which is just before the boss chamber. He had two phantoms with him and they were all currently engaged with several Shells at once. I soon realized there was only one way I was going to win this. I two-handed my weapon and rushed straight towards the host. Zoomed past the two phantoms before they realized what was going on and stabbed him right in the back! He had sustained some injuries from the demon oysters prior, and because of that, died then and there.

I defeated a host who had backup when he was a mere skipping stone away from the boss, and did it looking like a crazed assassin! It was by far my most epic invasion ever!

Never playing WOW ever again, and i have no idea how this worked but getting a pop up on screen while playing starcraft 2 saying i got the cheat achievement truly proud of that.

Halo 3 achievement required 4 people to complete an entire mission on Legendary with the Iron skill on and all 4 of you on ghosts, the achievement was worth 0 gamerscore.

It took us 4 hours to complete it, it was nothing short of a pain in the [censored], but it was total worthy it and has to be the best experience/accomplishment of my gaming life

Ha I remember getting that achivement with some friends. Was fun, but dear god I hated the first part with the flood.
Also had to do it twice becuase someone lost a ghost iirc.

It could have been worse, it could have been cortana on legendry with iron.

After years of trying I managed to finish Hitchhiker's Guide to the Galaxy on the C64. That game was seriously punishing.

I also remember some maddeningly difficult missions on the first two Wing Commander games on PC when I was a kid.

I done it twice as well, a friend wanted in on the action so he could get Recon - never did.
The 2nd time was brutal because we let my sister play as a guest, she couldn't drive for [censored], and it must have taken us double the time because of distractions and the amount of deaths she suffered

We had some people who had issues driving too, but it made it more fun What was bad was the floor falling out beneath the person who was furthest back.

Still the driving part was much better than climbing the tower, those [censored] flood, especially the ones with shotguns.

I think I got recon (all vidmaster iirc) didnt use it though. Becuase by the time I got them the community was utter [censored].

The floor was a real challenge for driving on, my whole team were slightly delayed getting off the structures because of the eploding Flood guy, so by the time we got to the falling floors pieces were already starting to fall. But almost everytime with out fail either my sister or another boy would fall down this one hole, not matter how fast/slow or carefully they approached it. It got infuriating eventually for everyone. Great day though

I never got Recon, I think I was one mythic achievement away - on another note I've recieved achievements for that DLC yet I don't even have it installed, some of them had to be played on mythic maps as well which was odd .

Leading a once poor and primitive Hindu Kingdom to unify India and conquering half of China in EU3:

http://i485.photobucket.com/albums/rr212/Qeros/eu3end2.jpg

Took aaages.

I did that with about 5 characters I think. Not always by myself though, especially the first few..back in those days it was nigh impossible to do some of the missions with a full group of people. (I'm looking at you Thunderhead Keep). Now, with 9 characters I've made it to the end of Nightfall (to get the free chests), this is not really a feat per se..but the amount of boredom it caused me..well I've never been that bored in my life to do something and the fact I did it was the feat.

Anyways , I have a couple others from that game.

The first is being one of the first (I was in the top 3 if not the first) people to beat the Nightfall campaign after it released. I beat it Saturday morning and it was released Friday morning at midnight. I saw 1 or 2 people in the outposts at the end game so, I was in the top 3 at least like I said. I was the first to post about it on the unofficial forums though.

The second is, back before Factions was released..sometime in December of 05 or January of 06 I took my Warrior and fought..(not ran) solo, my way through Lornar's Pass all the way to Droknar's Forge. I didn't clear any areas, but I fought my way there and at that time it was much much harder, especially solo. It took me 5 hours, but I felt pretty great about it. Very few people could do that back then, that I knew of.

The last thing that isn't really something to brag about I suppose, I've got over 5,000 hours on that character, the warrior I was just talking about. Good times in the past with Guild Wars..hasn't been the same for me since 09 though.

Lol my manager and I did all 3 of those Moxxi quests and..it took us 4, 5, and 6 hours and well I know how you feel man..I really really do. We couldn't play the game for about a week or two after that, each time we did it.

Oh I have a couple more!

Completing Arena & Daggerfall, I remember thinking I would never be able to complete the main quest since I didn't grow up with those style of games.
Completing Fallout 1. Another game style I never grew up with, so when I beat it by diplomatic means, it really made me happy.
Completing Gears of War 3 on Insane, I never thought I would do it, it was so infuriating that when I beat the Lambent Berserker I was literally screaming and jumping around the room.
Getting all but two achievements on Halo 3 and map packs was awesome as well. It's a shame I never got the two I was missing though.

I'll probably think about a few more later.

It sounds like you underestimate your abilities.

Having 45 wins to 15 losses with Nature's Prophet in DotA 2 matchmaking.

Beated Final Doom: Plutonia Experiment on Ultra Violence. http://www.youtube.com/watch?v=wCGeNNvG0Z4 (not my video, i use a slower, less guns blazingly tactics ) They don't make games like that anymore

Of course that is easy, now beating http://www.youtube.com/watch?v=I8ZgtBl1sr4&feature=related on UV would be something to brag about.

Stopping Dagoth Ur and fulfilling the prophecy in Morrowind.

Getting all trophies on Demon's Souls and getting to New Game+7.
Beating the Lich King in World of Warcraft.
Becoming competent in SSF4.

Back in the day...
Getting all gold gems and all S-ranks in Mischief Makers.
Being able to beat Sigma in Mega Man X without using subtanks/beating the game within an hour.
Beating Majora in LoZ: Majora's Mask without the Fierce Deity Link mask.
Catching Mewtwo without the Master Ball in Pokemon Red / getting through that one cave that "required" Flash without using Flash.

Getting 100% clear on Timesplitters 2. Atom Smasher mission on hard mode =

And then my saved game was promptly lost on that same day when my cousin pulled out the multitap while it was saving.

One of the mega man games, There was a series of jumps on small moving pads all the while flying robots shooting you, falling off the pads was death and you had to redo the level to get to this point, finally did it.

Of coarse tf2 being on the winning team on the top of the list at the end of a game.

I got the Survivor title in Guild Wars before they changed it (it used to be that if you died you had to reroll if you wanted the title, now you just have to reacquire enough XP starting at 0 which is still challenging but not quite as much of an achievement as before).