The Korean automaker's rise to the top has been "fueled by its low repair frequency," according to CarMD. Toyota is second to Hyundai in lowest repair frequency, but Toyotas also have the highest average cost of repair. (Interestingly, General Motors vehicles have the lowest average.) Both Toyota's and Hyundai's manufacturer "health" ratings suffered this year, but Toyota took a bigger hit, which also is what helped Hyundai achieve the top ranking.
While Hyundai is the top manufacturer in the index, the 2012 Toyota Camry is the top vehicle. The best-ranking Hyundai was the 2010 Elantra (behind three Nissan models and four Toyotas), which took the number eight spot. Hyundai has six other vehicles in the top 100.
The top-ten manufacturers with the best overall vehicle "health" rankings are Hyundai (No. 1), Toyota (No. 2), GM (No. 3), Chrysler (No. 4), Honda (No. 5), Ford (No. 6), Nissan (No. 7), Kia (No. 8), Volkswagen (No. 9) and Mitsubishi (No. 10).
Check out CarMD's study (Warning: PDF file) to check out all sorts of data related to car health – there's a ton of information to peruse.