What is the best highway tractor brand?

Well, what's your criteria of "best"? As far as prestige, Kenworth, Peterbilt, Western Star (pre-97), and Marmon (no longer in production) tend to be the most highly praised in North America, while Scania and Volvo receive the same accolades where a long hood conventional isn't very practical (e.g., Europe, Asia, et. al).

For ease of maintenance, Freightliner has a lot of advantages in this regard - this would be particularly crucial for someone buying a used truck, but also for those buying a new truck once the warranty expires.

Did you want the best for an experienced owner-operator, a new owner/lease operator, an fleet staffed by experienced drivers, a fleet staffed by new and/or lightly experienced drivers? What's best for each of those isn't going to be the same across the board. Then there's the application. What someone might consider best for long distance line haul may not necessarily be so for someone doing local work with something like an end, side, or belly dump trailer. The suspension which gives the best ride on the road may run the risk of being too flexible when you have a frameless end dump raised up in the air.

So, as is, your question really is a bit on the vague side. What's best for someone will depend on their level of experience (or the level of experience of the person hired to drive the vehicle), their finances (a used $25k Freightliner will be a lot easier on the pocketbook than a new $120k+ Peterbilt), the application they use it for, and a number of other factors, including what their criteria of "best" is going to be.