It is a tough choice for me. Of them all, I have two that tie for first place. The first is Eafoth the Cleric, the other is Maelrigar Desraktu, an orcish monster.
Eafoth
(http://i200.photobucket.com/albums/aa238/Fumblez/th_Eafoth.png?t=1282608571)
was a pretty rad (wo?)man. S/he was a Radiant Creature based off of a human, losing 3 ECL to that template. They also had 8 Cleric levels, with the Sun and Strength domains. I figured that Pelor was a pretty good domain for a creature with bonus spells based on light, and only later did I find the Radiant Servant of Pelor. Eafoth quickly went from being the character 'only alive because it can provide heals so we have to save it' with a mere 40 hit points at ECL 10 with no real firepower to becoming an ECL 13 powerhouse with the ability to slay dozens of skeletons per round.
The best part? This particular cleric of the light was as blind as a bat. Literally. They could only see because of that fancy red headband in the character portrait, the Blindfold of True Darkness. It gave him/her 60' blindsense with which to see. The trick to all this, though, was that they were blind by choice. I didn't get any benefits to being blind, and I made sure it was this way because I never stated that their eyes didn't work. They just couldn't see with them. Do you see any eyeholes in that robe? I figured that being a glowing target was a bad idea in a world mainly populated by evil-aligned outsider-haters. Thus, nobody in my party even knew my gender until we ran into a pair of vampire wizards, levels 20 and 15. Most of the party is trapped by spells or the vampires' mooks within the first turn, and the only people I could see standing freely were myself, the heavily optimized swashbuckler, and the two vampires. (Technically I could only see the swashbuckler in the meta-sense due to their Darkstalker feat preventing me from auto-succeeding on the spot check.) I suddenly disrobe, and out of sheer dumb luck both vampires fail their saves to avoid being blinded. While they can't see, I unleash impossible amounts of damage via a couple maximized Searing Lights followed by my usage of the gauntlet I bought long ago with two charges of Anger of the Noonday Sun 2/day. Because I became a Radiant Servant of Pelor, the radius of that second spell extended so far and did so much damage that nearly all of the mooks died. My party swiftly finished off the blinded and heavily wounded vampires, and by the time it was all over, I had already reassumed my robe. They never guessed what happened, though my DM gave me a high five afterwards for how awesomely I did against the vampires; he was expecting a near-TPK, and instead I flattened them in a matter of rounds. Best individual fight ever.
Maelrigar Desraktu
(http://www.danddminis.com/assets/images/King_Obould.jpg)
That right there is a pretty fitting picture for Maelrigar, the level 4 Water Orc Battle Sorcerer. Originally created to be extremely weak to balance me with the rest of my party (they were all new guys), Maelrigar far exceeded my expectations. He was unnaturally good at melee combat, and in fact several of the other players thought he was a fighter for the first dozen sessions or so. Being a Battle Sorcerer, his spell list was very limited, but +6 strength from Fist of Stone goes a long way. Here are some highlights.
1) One of the party's first adventures was to visit an abandoned tomb where a necromancer was said to be desecrating the dead within. Wandering through the halls, we slew a few skeletons, nothing really important. Suddenly one of my party members decides to open one of the doors from the main hall, and from within he hears chanting. Like a reasonable human being, he slammed the door shut so that the party could organize and fight whatever came through. Like the unreasonable Water Orc I was roleplaying, I decided that the best course of action was to stop the ritual before it could be finished. I bull rushed through the door, and somehow my wild guess of an angle was exactly correct, giving me the ability to push the door onto the necromancer and break his concentration. That being the surprise round, we had yet to roll initiative. I rolled at the very top of the list, and immediately got a critical hit with my scimitar, instagibbing the boss of the dungeon.
2) Later in that very same dungeon, the gnome of the party attempted to move a stone slab by pushing on the bottom. Due to the un-luck of a d100 roll, the slab started falling towards them. On reflex, Maelrigar tackles the slab aside and smashes it against the far wall.
3) The party heads eastwards with a caravan of goods. A band of orcs attack, aiming to steal the wagons and horses. The druids of the party entangle the ground all around the caravan, hoping to slow the orcs. Our DM being of the sadistic variety, the orcs were master crossbowmen all along. Maelrigar has no ranged spells, and he doesn't want to get trapped in the entanglement. Instead, he draws a couple knives from his belt and throws one every round. He isn't very good at this, but there is little else he can do. Suddenly, after half a dozen misses, he hits an orc. A critical hit, killing the orc instantly as it reaches for the first horse's reins. Another shot, another critical. This happens not once, not twice, not thrice, but four times. The DM even made me change dice every time.