There's really no ONE "best" hillside combine. They are all very good, as are regular combines. Yes, Case-IH and Deere are the most popular in hillsiders, but that's just because they are two leading manufacturers of combines, period.
Due to the rising cost of the hillside combines as opposed to their flatland counterparts, the manufacturers have all quit building their own hillside features and have gone to contract manufacturers such as Hillco or Rahco. Both are very good makes, too. No, these are not just some after market add-ons the new farmer or harvester owners install. The combines are sent out to the respective companies for professional installation. This stuff HAS to work; otherwise some person could die! Both Hillco and Rahco work jointly with the combine company to do this.
Yes, I've seen other combines [pre-owned] have these hillside features added. I can only presume in such a case, the dealer or even individual makes all needed arrangements whereby the mods are made at the respective factories.
Still, back to your basic question about combines themselves. I truly believe that since all combines are built to be the best money can buy, then it is up to the OPERATOR to decide HOW WELL to set it, run it, maintain it and KEEP it running at its best for many years to come.