"I don't always play flash games, but when I do, I prefer
Coign of Vantage." Haha, but seriously... I think its uniqueness is what makes it so likable. There's pretty much a cloud of pixels at different depths, and you have to rotate the playing field so that the vantage point--er, coign of vantage, I could say--matches the image in the upper right corner. It's a simple concept, but it's rather addicting.
One other flash game that I used to play a lot when I was in eighth grade, was
Bloxorz. It's similar to the Sand Wand puzzles from
Spirit Tracks in a lot of ways, but you pretty much have to get the small end of your block into the goal--while trying to use the least amount of moves possible. It can also be pretty addicting, though, it gets pretty frustrating around the last few levels.