other than the one that came with the camera.... Hmmm....
Well first camera was a Practika POS which fell off a mountain (and good riddance, if it hadn't had a irreplaceable roll of film in it), Second was a Canon AE-1 Prog with 50/1.8, and the first add-on lens I bought was....
A Canon FD f2.8/28mm which i've still got, and which is doing sterling service on my recently acquired A-1...
bought because at the time I was shooting mainly landscapes/climbing pictures and needed something with a bit of width about it.
If you're talking about modern stuff though, it was a Nifty 50. Bought because I had always been used to fast glass before going digital, and it was the cheapest way I could get some and still retain AF. Unfortunately, I forgot about the crop factor, and to be honest, it rarely gets used now, apart from the odd occasions when I agree to take some photo's for my mates band, or on the EOS-3 film body. Good job it was cheap. I'd quite like a 35mm f/1.4L, to do the job I'd originally fancied, but not at the price they go for!