At one point in time, I could easily 100% Metroid Zero Mission, but that game isn't too hard to cap.

I've completed Castlevania Aria of Sorrow and Dawn of Sorrow, collecting every weapon, piece of armor, and soul. I also capped Castlevania Order of Eclesia.

Then there's Banjo Kazooie.

The most time consuming games to cap would probably have to be the Castlevania games. Banjo Kazooie doesn't require you to go out of the way to grab everything and Metroid Zero Mission was designed so you could beat the game 100% in less than 2 hours.

When it comes to flash games, the only one worth mentioning is Amorphous Plus. It is such an absolutely AMAZING game.

I'm a bit of a fastidious gamer, so I tend to try for 100% on every game in which the details are at least somewhat incidental to the main objectives.

I 100% every driving game I've ever played, which includes the Test Drive series, the majority of the Need For Speed series (and I had to go back multiple times and create a new save game to unlock the other unique gear, nnngh), several of the Codemasters titles like GRID, even arcade titles like Daytona (lol), Screamer 2.

Probably the largest shooter game I 100% would have been Jedi Knight (and Jedi Knight Academy), because in those you can get permanent force ability buffs by finding all the secret areas, and more force abilities is good. Not even going to try 100% Assassin's Creed II, or Far Cry 2. Finding all those feathers and diamonds is a pain in the butt, and as far as the latter goes I've already unlocked all the awesome grenade launchers and ammo upgrades, I'm like a walking tank. Will definitely try 100% for Sleeping Dogs though, that title's fun enough to try and I need money to buy all the cars.

I was 99% on Mario Advance, but try as I might there was one level in World 4 where I couldn't find the darn secret platform to get to the secret stage. I'd found it in every other place, and I spent like an hour (and several lives) whizzing up and down the stage in my tanuki suit and nothing. So annoying! However I did manage to find everything in the Mario Superstar Saga (must play again, was incredible fun), and power-levelled so hard that I one shotted the final boss lol.

One reason I don't play too many flash games here is because I usually try to 100% them too, and then I blow several hours...

Chrono Trigger, Super Mario World, Oblivion, Fallout 3, Diablo 1 and 2, Final Fantasy Tactics, Cave Story, Spore, and a couple others that aren't off the top of my head. Spore is not from friendliness; friendliness would take me forever. 100% Final Fantasy Tactics is harder than you think. . .