For the WSC Challenger class, this is not a difficult question. Nuon Solar Team owns the race, and has won six times out of eight this century (although “uneasy lies the head that wears a crown”). The more interesting question is: who is second? There are four main contenders for that honour.
A few years ago, I would have placed Tokai University second. They won the race in 2009 and 2011. However, unless they can reverse the trend, their star seems to be falling.
Michigan are very definitely the best US team. However, they have pointed out themselves that they suffer “the curse of third,” and thus far lack the je ne sais quoi that it takes to win (of course, when they find it, Nuon had better watch out).
The star of Solar Team Twente is rising. They worked their way up to second place in 2015. They could win this year.
Finally, the Belgian team from KU Leuven is also moving up, and I expect them to do very well this year also.