What is the best tractor trailer truck?

For what application? "Best" is a very relative terms, and what's best for an owner-operator with years of experience won't necessarily be the best for a company which recruits new drivers. Among OTR owner-operators, the Peterbilt 379EXHD, Kenworth W900L, and pre-97 Western Star 4964FX have always held the most prestige, and, to a lesser extent, the International 9900ix and Lone Star. For hauling heavy loads short distances and sitework, Mack has traditionally been the name for that role, though they're not so popular for line haul. Sterling has been a popular marque for daycab applications, both with straight trucks and tractor-trailers, as their daycabs are quite spacious. For European marques, Scania and Volvo are considered the most prestigious.