The ubiquitous 50/1.8. Whilst I really like the lens, I don't think it warrants the comments that everyone should have one. Its nice and provides great image quality for the price - heck, oits great image quality for three times the price - but most people won't be able to use one of these really well - me included.
The focal length is a bit long for me on a 1.6x crop camera, and for typically "short tele" shooting the AF is too poor, and the MF is really awkward and hard.
However, I still carry this lens in my bag. It is good for those dark indoors details shots - too long for people shooting inside - and coupled with a set of extenion tubes, makes a superb intro to macro. My best few shots have been taken with that combo.
So overall I like it, but it is a more specialised lens than people consider it to be