Well Vergil from DMC3 cuz i loved the theme, and the brother fighting, and it beying "the last battle".
I loved fighting breene in halflife, cuz i love the whole tension of trying to rush and beat him, and having the hair up by beying nervous....
i loved tzar last level because it united nations worldwide to defeat one common enemy....
i loved killing the last boss in kingdom hearts, because when everything seemed lost, sora arose from the shadows and fought till the last breath...
i loved fighting the boss in snake eater, cuz the song, the moment, the tension, fighting your mentoy, a person that isnt evil, but that sacrificed everything for her country (it was a very honorable fight)...
i loved defeating Zeus in God of War 2, the rage i had, i could figure out what to do and when i did, i went onwards to one hell of a fight... and i won...
i loved DeusEx because of the philosophy it teached, and the last moments of the game create a tension that will make you follow one of 3 endings.... i became a god...
i loved beyond good and evil ending, i think it was a very powerfull game and fighting that boss that made your keywords backwords was a pain in the ass... beating it was a great moment...
I loved beating pokemon blue when i was a kid... i felt like i achieved something, listening to that elite 4 music was epic, and i trained really hard for that...
i loved beating every single spyro and crash bandicoot cuz they are classics ill never forget...
i loved banjoo kazooei and problably every single n64 game, i loved that generation...
But allas after so many adventures, rescuying princesses from giant turtles, saving zelda over and over, defying armies in my metal slug tank, going through heaven and hell, fight aliens, mummys, zombies, nazis, zombie nazis, monster nazis, demons and creatures of the unknown, i will say nothing beats my first gameboy game i ever beaten in those days.. zelda dx... it was not the best of game boss battles, but i felt good because it was my first that i beaten, and i worked on that game for so long...