Wii:
Guitar Hero 3 / Rockband 1 - We put tons of hours into these, but I have no idea how many
WiiFit - I'm at over 30 hours over the past 95 days
Resident Evil 4 - At least 30 hours, probably more
Zelda TP - At least 25 hours, probably more

DS:
Picross DS - I've spent days of my life playing this...
Civilization Revolutions - Same as above, I've completed all of the possible winning combinations on the hardest difficulty level. I'm still addicted.
Advance Wars Series - So many battles
Phoenix Wright Series - These consumed a month of DS playing, went through them all straight, then started again :p

Voltaire:
If you play a game and save it it'll tell you the time.  I think it keeps counting if you continue playing again with the same save.

Wii - Smash Bros
DS - I have no clue. Possibly Pokemon D/P?

Without a doubt, my most played Wii game is Mario Kart... its something you can never stop playing and its still fun even after you've unlocked everything -- the online races keep will keep me busy for a whole day.

On the DS, there aren't really any great multiplayer games to keep me playing but I'm replaying New Super Mario Bros. right now.

Super Smash Bros. Brawl with around 150hours.

Wii - Guitar Hero III
DS - Puzzel Quest

Wii- Guitar hero 3
Ds- All games in the ace attorney series

DS: its Advance Wars: Days of ruin. Wii: SSBB.

Okami, about 150 hours. Then Kart and Brawl.

wii- smash bros. 140
ds- professor layton 35-40

Recently, it's been Animal Crossing: City Folk (about 40 hours overall since I got it) and Chrono Trigger (about 25 hours since I got it). Overall, it's Legend of Zelda: Twilight Princess (currently pushing 200 hours overall) and either Animal Crossing: Wild World or Harvest Moon DS (I'm honestly not sure which I've played more, but both are well over 100 hours).