Zeiss Conquest or Leupold VX 2 or maybe even 3i (which I think can be had in 2.5-8x for under $400 now).

To the OP. I've stated this so many times I know I sound like a broken record but the fact remains that:
There are only four or five lens makers globally. There is only one company I know of that grinds there own glass, there may be more. Vortex and Leupold(as an example) could be purchasing some of their glass from the same place.

Where I believe most of the differences come in are the quality(cost) of the glass and the coatings each company puts or has put on their lens.

That is why NO one can tell you what the best scope under $400.00 is for YOU. That is why you get so many opinions. I've looked through a lot of scopes in the last couple of years. I've looked through all of the scopes mentioned in this thread except for the SWFA line. Some of them I couldn't really tell a difference, for example a Vortex Diamondback and a Burris Fullfield II. That was in optimal lighting. When taken outside in low light then the Burris jumped out at me as being better. You may see better through Zeiss glass than Meopta, or Leupold than Zeiss. You need to find a store with a good selection of scopes and look through all of them. One or a couple will stand out over the others.

Other than mechanical quality, such as turret adjustment repeatability, I just don't see how one person can tell another which scope is better.

There are compromises too. I don't believe Nightforce has the best glass in its price range but I do believe they are about as tough as you can get. If my life was dependent on my scope, I would get a Nightforce. I believe Leupold focuses more on glass quality in most of their scope lines. All I need is for the adjustments to work well enough for me to sight a rifle in at 100 yards, that's it. So Leupold works well for me. If I did the type of shooting JGFIREMAN does then I probably wouldn't be using Leupold and would sacrifice some glass clarity for features and repeatability.

Often overlooked but any Bushnell with Elite in the name is a great scope as far as clarity and brightness.

If low light performance is what you want then I would steer clear of any of the vortex viper and under line.

My 20 year old Leupold 3x9-40 vxII's are better in low light than the 2 year old Vortex viper I have. The vortex has been good, but like others have said I bought it for the turrets not its low light performance.