Mario/Duck Hunt, my only game for ages.
Rollercoaster Tycoon, beat everything and I made something silly like year 99 in one of my parks.
Pokemon Red, My first save was about 80 hours. Some moron overwrote it. after that I got 151/151 so easily 399 hours there.
Pokemon Silver, 250/251 and I completed it twice before that savegame.
GTA: Vice City, (100% four times takes some time)
GTA: San Andreas, (100% two times)
GTA4, never 100%'d this due to GFWL deleting my savegame but I beat it twice and came to 98% so im sure I clocked 100
Fallout 3: Still did not complete this; just testing mods takes like 20hours then you start playing.
Team Fortress 2, Over 1000 and still counting.
Age of Mythology: the Titans, I had a "clan" and played it non stop for 6 months.
Age of Empires 2, see above.
Advance Wars 2, S rank all war room maps except 3, s rank Hard campaign. lots of hours there
World of Warcraft...yeah over 150 days / played combined on all my characters when I quit
Left 4 Dead 2, 130hours according to steam
Mass Effect, beat it 8 times.
I bet I forgot some. before steam sales I bought like 3 games a year and now I have a huge backlog and still stick to the same games.