For me I'd say Assassin's Creed II, Halo 3 and Sleeping Dogs.

PS3

Resistance 1 through 3

Uncharted 1 and 2

Demon's and Dark Souls

Ratchet and Clank Future A Crack in Time

inFamous 2

PSP

Metal Gear Solid PO and PW

Gran Turismo

Wii

Donkey Kong Country Return

Super Mario Galaxy

DS

Pokemon Soul Silver

This gen, uhmmm. Super Smash Bros. and Just Dance 2 :)

Dark Souls: The game is so epic that I put way too much time into it.

Deus Ex: Human Revolution: This game is so close to being a masterpiece. I loved almost everything about this game. The world is just so fully realized and we are missing stealth badly this gen.

Smash Bros. Brawl: Yeah, the name speaks for itself.

Killzone 3 multiplayer: I've played quite a few matches.

This gen: The Halo and Mass Effect games

PC Today: Portal 1 and 2

Last Gen: Halo 1, 2 and Skies of Arcadia for Dreamcast

PC 90s: Dark Forces II Jedi Knight, Half-Life and Quake

32/64 Bit Gen: Zelda OoT, MGS and Panzer Dragoon Saga

PC DOS: Doom and C&C

16 Bit Gen: Sonic and Mario games

8 Bit Gen: Sonic and Mario games

8 Bit Computer: Donkey Kong

Golden Age: Pong

Brawl for sure

Xenoblade

Monster Hunter Tri

But I've sunk serious time (50+ hrs) into
Mario Kart
Endless Ocean (1&2)
FE:RD
Godfather
GHIII
Wii Sports
Wii Fit / WF+
Mario Strikers

making them all well worth their price

Battlefield 3 above all. I've put so many hours into the multiplayer... and I still play it almost every single night.

Also, Dota 2. It's free to buy! And that's full price! And it's given me amazing value so far. =)

The Mass Effect games. 6-8 times apiece for the first two. On my fourth playthrough of the third, and an ass-stomping ton of multiplayer in the same.

Combined total playtime is huge

Add in however many hours my wife has logged in (she's played each game nearly as much as I have) and it gets pretty serious.

mass effect 2 and 3 over 100 hours
final fantasy 13 - 80 hours
skyrim - 100 hours
uncharted 2 and 3 - about 45 hours
assasins creed 2 - 25 hours

all of those were extremely more enjoyable than any other game ive played aswell as longivety

Gears 1 - 3: I've put countless hours into the MP of each of these making each worth every penny (and then-some).

Single Player wise Fallout 3, Oblivion and Skyrim. Over 100 hours in each. Hell I don't even know how I found that much time TBH.