I don't know which is harder, choosing the best or the worst!

Kane & Lynch is probably near the bottom - I forced myself through half the game and it was only bloody-mindedness that got me so far. Never any real fun, quite unlike the Hitman games... I could also nominate Spore, for delivering the most half-assed game modes instead of any deep gameplay and being a big, soggy disappointment. But the worst of all (that I've played on PC, anyway) would possibly have to be Medal of Honor: Airborne - there were just so many things wrong with that game, I don't even know where to begin (though Yahtzee put it pretty nicely.) When an indie/B-game developer produces substandard drivel you can usually take it with some good humor, but when an A-list team comes out with something as confused and pointless as Airborne it's just sad.
I would also say the first TMNT game on the NES/Famicom is a pretty decent contender for worst game ever, pleasing neither fans nor anyone else with any sense of fun... It reminds me a bit of Takeshi's Castle: a game so diabolically hard that it not only stops your progress dead, but also humiliates you for not having the superhuman capacity to beat it.
As for best game, that's actually harder, now that I come to think of it... There are a lot of games I like for different reasons, and quantifying them into some coherent order is next to impossible. Morrowind is certainly up there for its incredible vision, Half-Life for its clever redesign of an entire genre, Operation Flashpoint for pushing shooters into the realm of realism, Grim Fandango for being such a beautiful swan song for those LucasArts adventure days... Gah, I can't pick, so I'll settle for the Civilization series, which has gobbled up not just hours of my life, or days, but
years.