Bioshock/Left 4 Dead

I got Bioshock with my Xbox 360 in December, and I hadn't been very excited about it. Of course, I'd wanted it, but not as much as other games.
It's only recently that I've realized how freaking great it is. It's got to be the most fun and original game I've ever played.
Originally, I was a bit adverse to playing it because I found it so freaky. I was always afraid of everything in it, because I didn't really get it. But now that the shops are relatively common place and I can hold my own, it's less terrifying as it is extremely exciting.

I got Left 4 Dead recently. It's also a very fun game. The co-op aspects are great, but the plot is nearly non-existent. There are four plot lines each with five levels, but these can't really be called plots. Each one is different from the last, the first plot being a city/hospital, the second rural, the third city/airport, and the fourth rural/farming. Each one consists of you rushing through to the next safe room in levels one through four, while the last level (the fifth) has you holed up in a defensive position waiting for a vehicle to come and save you.
Each plot lasts for about an hour, if you take your time. I've played all four plots on normal, so I tried playing on Expert for the achievements. There was a mistake.
There is a serious difficulty gulf between normal and expert. This is, of course, only to be expected. It took me three tries to get through the first level in the first plot, and I gave up after dying a second time in the second level. Obviously I'm going to need some practice before I can take on Expert, which should keep me playing for a while. (Probably only after I finish Bioshock, though.)
I also expect more replay value once I get Xbox Live, but I'll probably be playing other games like Halo 3 on it for a bit first.