I played HEAPS of really great Flash games, but the ones that stand out the most are the Exmortis series, which was a nifty horror-themed flash game, and The Last Stand, which actually has a great roguelike game on Steam now loosely based on the old Flash game versions.

The first Exmortis was a relatively simple point and click game that you could beat in probably less than an hour, but the visuals were amazing for a Flash game and it scared my socks off the first time I went through it. And it gets much deeper with Exmortis 2 and pretty crazy with Exmortis 3. And The Last Stand had great replay value and was one of those games you could enjoy playing again and again.