Tokina 50-135 f2.8
This lens is very sharp straight from f2.8, so it is a great choice for low light shooting if you want a zoom. I find that it is a better lens for portrait than landscape work, -- just based on the experience. Flare might be an issue. The biggest drawback -- sluggish AF, I wouldn't use this lens for anything that moves fast. Other than that -- it is a real winner IMO.