The worst game I've ever played (that wasn't free- I've played too many bad free games to even count...) has definitely got to be Oblivion...
Combat is lame, the engine is buggy, the game is terribly optimized (it runs terribly on my PC while UT3 runs at max settings insanely smoothly) without .INI tweaking...
Sure, there are numerous quests, but they are all LINEAR. As in, I can't be an evil villain unless I join the Dark Brotherhood, but even at the end of that questline, only the members of the Dark Brotherhood revere and fear me, while everyone else STILL asks for my help and treats me like dogmeat. What the hell?
And don't even get me started on the combat. Unlike Dark Messiah of Might and Magic, Oblivion's combat lacks any sort of tactical elements. You just spam the left mouse-button as a fighter, assassinations aren't real assassinations (and the stealth system is totally broken), and magic is boring as hell, with spells that make me feel like a weakling, instead of an all-powerful badass.
Oh, and the traps are also broken... apparently, getting squished together by two stone walls doesn't turn you into a bunch of pulp, and it's impossible to actually be IMPALED by spikes.
The dialogue... oh, the dialogue... it's so bad that it's laughable. I'll admit, some characters are interesting and varied (like the Arena Blademaster), but for the most part, everyone sounds the same except for some key characters (notably the Emperor and Martin). And how come the old beggar that asks for money sounds like an old woman then starts speaking like she's 40 years younger? And what the hell happened with radiant AI? All I see are people standing around at shops or wandering the streets aimlessly, then going to the tavern to talk to eachother for an ingame hour or two, then going home to sleep. Where's the people talking about quests I can ask them about by ACTUALLY OVERHEARING them talk about the quest beforehand? Where are the NPC's with low morals stealing food from shops and other NPCs? Where are the NPC's hunting for food?
I rest my case.