NA KA because it's cheap and gets the job done. Basic I/H/E, bump the base timing a bit, a few free mods and it's at ~150whp/tq give or take a few. Not a whole lot, but enough for me to learn on and more than I can currently use effectively at an autocross or around the track. I could also say "because it came with the car" but the engine that came in my car developed a nasty rod knock, and after a lot of thought (and staring at my empty bank account) I bought another KA to replace it.

Now, for street driving it totally sucks. My real choice would be an LS2/T56 combo with an L92/L76 setup, a healthy cam, a tune, maybe some rod bolts to make me feel better about spinning it to 7k. Easy 470whp+, awesome torque, great sound (with true duals and an x-pipe to kill the LS "truck" noise). Probably $10k to get it done, about $9500 more than I had to spend, so in goes another KA.