It could be beating a certain boss, level, or even a game, whatever you want.

For me, I'd have to say beating Final Fantasy VII 100% complete. I got every singe materia and master them all, I maxed out all the stats of every character, I have all there ultimate weapons and maxed limit, I did all the side quest and even breeded a gold chocobo, there was literally nothing else that I could do in the game, with well over a 100 hours into it.

So there's my greatest achievement how about you?

Seriously 2.0

When I finish it... :)

edit: now that I thought about it... I think beating Super Mario Brothers..

Because it was the first game that I ever finished

Beating the Little Mermaid on NES.

Alternatively, outrunning Cheetah in World Class Track Meet.

I remember when I first beat Dizzy on my speccy. I was so chuffed.
Nowadays I think my best recent achievement was getting both 'Brass Balls' and 'Little Sister Saviour' on my first run through Bioshock.
I'll think about it and get back to you on my all time greatest.

- Beating Mucha ALeste on Sega megadrive on Hard mode !! it was tougher than tough.
- Removing the Vampire Curse in Oblivion,, took a lot of time
- Beating FFVIII, it was my first RPG ever played, so it was very hard to beat it, especially the part where you have to fight Ester (i guess) where he's got Rinoa on him and draining her,, the tricky part is that i was using only one save, and when you save before him, u can never go back to the outworld to level up, so it took me almost 2 hours to beat him, and more than 10 times trying.

Mhh.. that's hard. Maybe beating Jazz Jack Rabbit.. or the Water Temple in Ocarina of Time.

Mh, but nothing of all this has been as hard ( or tiring) as getting Command Rank 5 on PlanetSide, the world's 1st true MMOFPS by Sony Online Entertainment.

And now they basically killed my server. I hate SOE :[

Beat Battletoads...without game genie.

Being the first and only person to ever be killed on a non-PvP server in an MMMORPG. Found a glitch that a buddy and I tested out. He killed me, and it was broadcast over the entire server. Needless to say, 4 GMs and 2 programmers logged in IMMEDIATELY after. =)

Getting a sick score on a level in SSX tricky when video taping it and send it to an UK PS magazine to become number 4 in their top 5.

That was actually the first game I beat without cheats or a Game Genie.