My favorite mod that I was most pleased with over the life of my old 6th gen was BY FAR the PSPEC. I was one of the first to have the revised one and it was incredible, I can't say enough good things about it.
up to this point it would have to b my coilovers cause wen i got them i had a blown strut adn the onli way to get it out was to cut the damn spring.... it took 2 days to install everything cause i tried everything under the sun to get the shock out w/o cutting the sping
for me it had to be blackhousing headlights cuz since i bought this recent accord when i tool the bumper off it was for the first time on that car lol and my worst mod was on my last accord i made a m3 trunk lip into a rear window spoiler type thing i even have a thread for it lol
edit heres the thread lmao http://www.6thgenaccord.com/forums/showthread.php?t=22650