Pokemon after the 3rd gen was my most played thing in the world, spending hours, and hours and HOURS at Pokemon Pearl was such delight :3 And also the Ratchet and Clank Series after the first game to the last PS2 game had my most replay value.
Most played would probably go to Mischief Makers. I had two files - my test file had over 42(?) days logged while my file with my best times had 25 days logged if I recall. They keep track of your full play time (seconds, minutes, hours, days) and I feel this is quite a bit of time to put into a platformer with only 5 worlds XD
I've probably put a lot of time into other things over the years like Tetris on the gameboy (owned it since late 80s and only recently stopped playing a lot once the 3DS came out) but I have no real way of tracking the hours on that. It'd be a lot though.
I'd imagine I've put more than 1k hours into FFX over the years (I've maxed the Sphere Grid - as in clearing nodes and putting the best stats in) 3 or 4 times in the NA version and twice on International. I've definitely put more than 1k hours into FFVI (the first game I fully bought myself and the first RPG I thoroughly explored - doing oddities like grinding to level 99 before Mt. Kolts for example). Mega Man series as a whole gets it, too.
Someday I would like to hope that Street Fighter sits at the top of my list and I will have developed the skill to prove it XD
Find me in the IRC and challenge me! I'd love to play against you!
Currently Playing: Persona 4 Arena