A team of twelve guys spent four years in creating this immense overhaul mod before they released it to he public for free. With all the video intro and menu, new graphic options, a self-updated installer, deep lore history, self-composed music, hand crafted weapons, landscape and characters that had 5x times more voice actors than the voice cast of Oblivion itself.
http://www.youtube.com/watch?v=QGkqjabgVks
2. The Morrowind multiplayer mod.
Never got someone else to play with me since the public servers were empty at the time.
But for someone to even think about making a mod like this into reality... and then go on making it reality, is insanity. Yes the mod was highly unstable and buggy, you could only stay in one cell at the time, and only offered some easy trade and combat options, nevertheless - it is one of the most impressive things I've ever seen done in any game modding community up to date.
http://www.youtube.com/watch?v=TdbRj4CREJ4
3. Morrowind Overhaul.
These modders made the game more beautiful than the game developers themselves managed to make it. Pretty damn impressive if you ask me!
http://www.youtube.com/watch?v=2r6hilB6Dlc
4. [snip].
What's your list?