RE: Swarovski, Leupold, Nikon, Zeiss?
If your main criteria is optical quality, then the normal "rank" IMO is Swaro Z6, Zeiss Diavari and Schmidt and Bender. Down a tiny bit are the Kahles "C" series and Swaro AV. That's certainly very important, but other factors that need considering include reticle type and design, reticle in what plane (1st or 2nd), tube size and length, objective size, exit pupil, weight, price / value, durability and warranty coverage. Factoring all these together you'll come up with the best compromise. I recently did that for a new rifle and selected the Swaro AV 3.5-10 X 42 with the TDS reticle.