Operation Flashpoint tops my list ...
I have to say Operation Flashpoint was a chronic time-eater for me. Definitely better and far more strategy than much of the genre that followed, such as the Battlefield series. But about 2-3 years before Operation Flashpoint, there was a little-known (and rather stupidly named), initial "dive-in 1st/3rd person" real-time strategy game called Wargasm from DiD that wasted a bit of my time too (although not nearly as good as OF). I have a retro Windows 98 SE gaming box with a Voodoo3 just for Wargasm and a few other games.
I really enjoyed the Elder Scrolls series starting with III: Morrowind. Wasted a bit of time on that on the Xbox, then bought the PC version and added in various mods. The main reason I bought a Xbox 360 was for IV: Oblivion. I just bought the PC version when Amazon.COM had it on-sale for $25 and $10 more for the Knights of the Nine add-on and plan on wasting some time around Christmas playing it all over again on the PC.
Going back earlier, I was big into flight simulators. It started with old Sublogic Jet (and not just MS Flight Simulator), the Microprose F-15/F-19/F-117 series, and them more realism in the Falcon series and a few of the F-22 products, including DiD F-22 ADF which was really a deep strategy flight simulator with its AWACS focus and then add-on/separate release (and I still play it on my retro Windows 98 SE gaming box). Probably the best right now is Lock-On: Modern Air Combat (LO-MAC), which is the latest version from the developer of the Flanker series (long history there, originally developed behind the iron curtain), and it really pushes the graphics.
The Origin Ultima series was great. Definitely liked the Origin Wing Commander products as well. Great stories and lots and lots of hours. I think I wasted a lot of time on Ultima II and Ultima III. (I have Wing Commander III, IV and Prophecy on my retro Windows 98 SE gaming box too).
I think everyone wasted time on Doom/Quake and my wife and I used to veg early in our marriage on some of the original Quake add-ons in multiplayer. Duke Nukem was just damn funny, throwback to the Army of Darkness quotes, and the Unreal series is always the "most playable and fun" these days. Especially if you have an older computer with an early GeForce series, get at least the original Unreal Tournament.
More off-the-path, I really liked the Decent series, including not only Decent III, but the Decent Freespace series of space simulators which were better than the Wing Commander series in my opinion. If you haven't played the Decent Freespace series -- I think they run on XP (they do on Linux since the engine has been released as GPL by Volution) and are $5.
In sports, I really mainly play NCAA Football and can waste a good amount of time running a season on that. I also liked the short-lived, ESPN-branded Sega NFL 2K5 product -- which was only $19.95 and looked a heck of a lot better on the Xbox than Madden. But, alas, EA games now owns exclusive rights to NFL and there was no NFL 2K6 product.
