1. Civilisation. The original on Amiga
2. Railroad Tycoon. Again, on the Amiga
3. Lord of the Rings Online
4. R6: Rogue Spear
5. Rome Total War

Sims probably aren't far off the next positions: Falcon 4, RFactor and European Air War all got a good 200+ hours, as do BFBC2 and Empire Total War.

1 Nascar Racing 2
2 Europa Universalis
3 Hearts of Iron II
4 Dirt Track Racing
5 Civilization IV

My estimated lifetime sim-time commitments

1. Grand Prix Legends
2. IL2 FB -->IL246 4.11

It is doubtful anything will have the chance to catch up to the time I've spent with those two.

3.GTR2/rFactor
4. Janes FA-18 (have not played in a long while, but logged A LOT of hours)
5. Red Orchestra or the Silent Hunter Series (a toss up because months will go by without touching them, then some months and thats all I play.)

1.IL-2 Sturmovik
2.R6: Raven Shield
3.Operation Flashpoint CWC
4.Ghost Recon

IL2 Sries
EAW
CoD the original + United Offensive , was my introduction to online FPS.
V8 supercars, me and my boy put in a lot of time beating each others best laps by tenths.
Panzer General series

First two spots go to MMOs for me, Asheron's Call and World of Warcraft. For non subscription based games it looks like this:

1. Warcraft III/Frozen Throne
2. FPS Football Series
3. Player Manager on Amiga
4. Geoff Crammond's Grand Prix Series
5. Red Baron

Honorable mention for "most Schadenfreude per minute" goes to Lemmings.

1. Solitare
2. EverQuest
3. World of Warcraft
4. Diablo
5. F-19 Stealth Fighter

Crusader: No Remorse. Hands down. Followed closely by EF-2000 Version 2.0, Falcon 4.0, and Total Air War, in that order. And then Lock On/Flanker 2.0/Su-27, followed by IL-2 Sturmovik and Close Combat.

I totally forgot about Crusader No Remorse. What a great game that was.

http://www.gog.com/en/gamecard/crusader_no_remorse

No pity. No mercy. No Remorse.

..another day another death...another sorrow..another breath

1. Operation Flashpoint: Cold War Crisis/ArmA2/Operation Arrowhead I count these as a single title, for the purpose of this thread, as each BIS title has led nicely to the other, although I did not play ArmA much before ArmA2 appeared.
2. Navy Strike I have been playing this one since 1997
3. F1 Challenge '99-'02 (particularly, an F1 2007 mod)
4. Flight of the Intruder FOTI was my flight sim of choice until Rowan's Navy Strike came along.
5. Great Naval Battles: North Atlantic 1939-1943 I still play it.

I am pretty sure that is accurate.

Ghost Recon probably takes the prize here- but I haven't had enough of a chance to let DCS A-10 catch up. Give it time.

EF2000
Falcon 4
I-War 1 (& 2)
IL-2

IL2, Grand Prix Legends.
To this day nothing has equaled the time I spent in those 2.
I'm less interested in gaming/siming than I used to be, so while I still get some stick time here and there, nothing is likely to every come close to these old favorites.

Asheron's Call: 700+ hours from 2000-2004
Battlefield 2: 566 hours from 2005-2011
Command and Conquer Red Alert: 100 hours
Battlefield 3: 90+ hours
Falcon 4.0 Allied Force: 90+ hours
Command and Conquer 3: 50+ hours

Jane's WW2 Fighters
Microprose M1 Tank Platoon
Novalogic Black Hawk Down
FSX
Armed Assault

I've got almost 600hrs into BF3 and that probably doesn't even touch Ghost Recon or Rainbow 6.

So:

1) Ghost Recon (+ Desert Siege & Island Thunder)
2) Rainbow 6 (+ Eagle Watch)
3) Battlefield 3
4) Rogue Spear
5) Longbow 2

Honorable Mention: 18 Wheels of Steel: Haulin' - 281hrs.

The hours my dad and I would play Ghost Recon over the home LAN is sick.

Janes F-15 Strike Eagle
Falcon 4
MSFS 2004 ACOF
MS FSX
Anno 1404

Probably Star Wars: Tie Fighter.