I second (or third) the recommendation for the 50 f2 Nikkor, though I wouldn't compare it to a Summicron unless you always stop down. The 1.4 is a decent lens if you need the speed, but suffers a bit more from focus shift, which is aggravated a bit more since you are focusing at 1.4.

The 1.8's are very good also, but zingier image quality; some people may prefer that.