Asahi is my fav - though many people find it very very bland/without distinctive taste. I think it is the ultimate 'dry' beer - so easy to drink, and great with food (not filling). It's kind of a 'soft drink beer' - sort of in the style of american beers (but something they fail miserably at).
Second favourite would be suntory malts (particlarly, on tap), closely followed by kirin original. Sapporo is another very nice brew.
One thing to be very careful of is happoshu (what I like to call 'fake beer'). This is something that was created to get around a beer tax. Basically, traditionally brewed beer attracts a certain tax, so they created a 'chemical beer' - unbrewed, 'beer flavoured' water (essentially).
It's identifiable in two ways - it never has beer written on the can (hard to spot if you can't read katakana), and it is significantly cheaper than real beer (for example, 180yen compared to 280yen).
Why avoid it? Well, it doesn't really taste like beer (some people wouldn't notice the difference I guess - so let's just say, it tastes like a bad beer!), and worse - it provides a killer hangover (way out of proportion to the amount you drink). AVOID!
more pics later tonight