The most difficult game you've ever beaten.

Vanquish is up there for me, along with God Hand, Dante Must Die mode on Devil May Cry and Ninja Gaiden.

Ninja Gaiden was tough. I enjoyed it is far as I got which was only as far as a helicopter battle near a train yard. Trying to down it with explosive arrows was not fun and I never did it. End of story.

Still army men land sea air. It's more that it has poor controls and doesn't use analog sticks, but it's still a bastard.

Without difficulty battle arena toshinden is out the window though!

Over G on HELL mode. I don't even like thinking about it. Or even busting up Raam on insane. Both of these had me pulling my hair out.

http://www.xgenstudios.com/play/plazma-burst

play this on hard

I *** LOVED Starwars: Pod Racer on the N64. That took me quite a while to beat, it was really hard in places.

Finishing Dungeons of Dredmore with a random build (I was really quite lucky with the skills I rolled) was pretty entertaining.

Devil May Cry was evil. DMC3 was brilliant, but tough as well.

I think my two crowning achievements have to be compleating TIE Fighter at about age 12 (what's that? You guys have twice the number of lasers, torpedo launchers, tough hulls and SHILEDS and I get destroyed in two hits? Why yes, I will be spinning through the void like a potato in a washing machine and screaming my childish lungs out) and compleating a game of Civ IV on Immortal or whatever the level below Deity is. I once nearly won on Deity as well, but then got pipped by some spanner winning a culture victory at the last second :-/

Well I never could beat Megaman X7. That was Axl's debut and I could never beat Red try as I did. The hardest game I beat was probably not actually a game but rather in MTG when I beat our best local player who plays a competitive Delver deck with my humans that I had assembled by trade and pulls whereas he bought his deck. So satisfying.
Edit: I really not played too many ridiculously hard games. Even Dark Souls wasn't too bad.

hardest game for me was eve online.

trying to convince people that you're something you're not for months, constantly being aware of how you act and what you say, finding loopholes to avoid any paper trails to link your accounts, presenting yourself so that you look like you're the only active account, slowly working you're way up their corporate-esque ladder and getting people to give you keys to doors, keys you really shouldn't have... and then robbing them blind.

that's hard.

I think to actually do a run through of spelunky without shortcuts is next to impossible. I love it but i think its the most hardcore game with basic mechanics. Even super meat boy doesnt have the two fingers up permadeath.... Binding of isaac looks super hard but i aint ever played it.... Permadeath is awesome but it really adds allot of difficulty unless you play better under tension
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I think they should have a server in dark souls that's pure perma death.... Imaginne that...

ok i thought long and hard, but i have a proper answer.

the hardest game i ever beat was metal gear.

the reason i say this. and the reason i compare this game to dark souls so much. is they are both incredibly technical games. Any yes i admit. if not for the help i have gotten from the forums and the wiki, it would have taken me much longer to trudge through dark souls, i would have beaten it probs eventually, but it would have taken forever. metal gear was the same way i spent so many hours in that game just with no clue what to do, it wasn't until a friend of mine got a password off someone near the end of them game, then he played the game backwards, and eventually we figured out how to beat it.

and as terrible as the metal gear games are today, the first was really the first modern video game. it invented things that are now common place in virtually every game you play. i really give it credit, and am willing to call it one of the best games ever made. and the hardest i have ever beat.