The obvious two - Oblivion and Fallout 3. OP armor/weapon combos, rather poor storylines (though F3 is waaaay more fun than grinding in the Plane of Oblivion, which after you get up to the more-than-basic levels can be REALLY hard with extremely strong enemies). Graphical glitches. I bought Oblivion for PC in January '07, then I was getting the weirdest sound glitch that rendered the game unplayable (couldn't find ANY help on it either ANYWHERE) because all I heard was this constant fluctuating wind noise. And I tried everything, emailing Bethesda, and reinstalling on a different harddrive (as well as re-downloading a few times) and I STILL got this problem. It eventually seemed to happen every time I tried to play the damn game. So I got it for Xbox and it of course has been much more smooth sailing. I must have had 3 or more PC characters and have sunk HUNDREDS of hours into the Xbox versoon. Same deal with Fallout 3. Huge time sinks. Also Oblivion expanded my horizons to games with sword fighting, as well as getting me interested in ancient history (already was a WWII buff) which I thought was really boring until I played Oblivion.
MW2. The only game I can play with friends really. Even though its easier to list all the UNDERPOWERED things in the game over the overpowered, I can see myself eventually going lvl 70 10th prestige.
Mass Effect 1. My 2nd favorite game of all time (ME2 is my favorite and is MUCH LESS flawed.) The battle system wasn't the best (I thought it was more fun than most people did but especially early on, it was really hard and not very fun). Disgusting texture pop-in up the kazoo, and very little DLC. Huge time sink (as is its sequel).
STALKER series. Another time sink. All 3 I liked despite their flaws. Bugs, stuttery gameplay and the end of the first two were so stupid that I never ONCE completed them. SoC's ending is much better and actually fun. And I didn't mind that the only weapon that was every really added was the Protecta in SoC.
Bad Company 2. It runs like absolute dogshit on my PC unless I turn it almost allllll the way down, has the signature horrid DICE server browser that actually has settings that work OPPOSITE of what they were intended (the Punkbuster filter), Rush mode seems biased towards the attackers, Conquest is basically TDM, the flags serve little purpose, and Squad DM is basically "Get shot from every single possible angle" and death usually from behind or the side. And of course the M60 trolls me to no end. And the sniper rifles need to be able to kill in one shot in the upper chest, and the only one really worth using is the M24 or the M95. But STILL. Its fun!