Here's my personal top 10 in order of difficulty (10 easiest 1 hard as all hell)

10 - 121 Stars in Super Mario Galaxy with both Mario and Luigi

Pretty easy, just had to master jumping with Luigi, the last few purple coins are the only frustrating parts.

9 - Beating Zelda II: The Adventure Of Link with only 7 game overs

Iv played this game 4 times now and every time my total number of deaths goes down by about 10.

8 - 100% completion in Donkey Kong Country and Donkey Kong Country 3: Dixie Kong's Double Trouble

I could 100% DKC 3 since I was a kid, DKC I only recently got but found all the bonuses pretty easy. Only DKC 2 to go.

7 - 100% completion in Metroid, Metroid: Fusion, Metroid Prime, Metroid Prime: Hunters and Metroid Prime 3: Corruption

Pretty easy, just got to remember where everything is.

6 - Completing Virtual Boy: Wario Land in one sitting (3 hours)

Gave me a nasty headache.

5 - Beating Super Mario Bros.: The Lost Levels

After beating this, I consider myself a master of 2D platformers.

4 - All trophies in Super Smash Bros. Melee and Super Smash Bros. Brawl

Boss Battles on intense still gives me nightmares, I staid up all getting that trophy. (Thank Meta Knight)

3 - All cheats unlocked in Goldeneye 007

As a kid I thought some these cheats were impossible, now I get em all pretty easily.

2 - Having some AX racers unlocked in F-Zero: GX

That's right, I only have 3 AX racers unlocked so far and have beaten to GP's on the hardest difficulty, and I'm very proud of that feat.

1 - Unlocking everything in Time Splitters 2 and Time Splitters: Future Perfect

Getting all the platinum trophies in these games was the hardest thing I ever did, but I did it.

i got 101% on DK64 and that game had a lot of shit to do in it,

double dang dude

You're bragging about getting 100% in Donkey Kong Country?  'Cuz you didn't find the hidden 1%.  That game goes up to 101%!

I got 101% I couldent remeber what the number was so I put 100% and I think I got 105% in DKC 3

What does DK64 go up to.  Now for some reason I think i remember it going to 102%.  Whatever it was, I completeed that game in its entirety.

I only played DK64 for a few minutes actually. No idea.

9999 VR in Mario Kart.

I've finished Super Turrican, Hagane and Wild Guns. They weren't that hard but it's the last time I remember having a hard time beating a game.
God Hand was pretty brutal at times though. Specially on hard.

This list is bizarre - No Fire Emblem? No F-zero? No full completion of Brawl?

SMW and SMB3 but no mention of the Japanese (ie proper) version of SMB2

Orcarina of Time? It was one of the best games of all time, but not even close to being the most difficult. Infact, of the home console zelda games, LoZ, LoZII, LoZ:ALTTP, Twilight Princess are all harder, Windwaker is about on par and I never played majora's mask