At the iPhone 6s launch event, CEO Tim Cook declared his phone to be the best device for world travelers. We'll go even further and say that it's the best for travelers within the U.S., too.
The reason is frequency bands. The 6s supports more 4G LTE frequency bands than any phone we've seen before, including the previous iPhone 6 and the Moto X Style Pure Edition, which were formerly the leaders. Supporting more frequency bands helps it connect to more networks for better coverage. It's like being able to tune to more radio stations.
Most notably, the 6s adds:
Band 12. This significantly improves T-Mobile in-building and suburban coverage and enables roaming on several small rural networks that use this band.
Band 30. This will speed up AT&T service in the future, although it won't add coverage.
Sprint carrier aggregation. This is going to seriously speed up Sprint performance in many areas.
This is complicated a bit by the question of antenna quality. In my experience, the iPhone 6 isn't as good as the Galaxy S6 at capturing weak T-Mobile and Sprint LTE signals. But adding these bands is really going to help.
For global roaming, the 6s keeps pace with the iPhone 6, which was already the best global roaming phone. For the best LTE roaming in Europe and the Middle East, you need bands 1, 3, 7, 8, and 20. For Japan, add 18 and 19. For Taiwan, add 28.
A bunch of other Middle Eastern and Asian countries are on 38, 40, and 41.
The U.S. versions of other popular phones like the Galaxy S6 and LG G4 tend to fall short on these points. For instance, the Verizon Galaxy S6 only has LTE on bands 4 and 13, allowing for no global LTE roaming at all. The T-Mobile and AT&T versions are better, but they lack 18, 19, and 38-41. The LG G4 doesn't have 8, 12, 20, or any of the high numbers. The new Moto X and Nexus 6 come closest, but they lack 18, 19, and 38.
What this means is that when you go abroad you're more likely to get 3G instead of 4G, even if 4G is available in your location.
That's fine, but it isn't optimal. The iPhone 6s has the best array of bands to capture the most global networks.
Alas, Not One iPhone for Everyone
There are still two models of the iPhone 6s, unfortunately. With the iPhone 6, the two models were "Sprint" and "everyone else." This time, it'll be "AT&T/T-Mobile" and "everybody but not totally AT&T."
One model (numbered A1633 or A1634) has AT&T's new band 30, but lacks the CDMA radio needed to make calls on Verizon and Sprint. The other (A1687 or A1688) supports all four carriers, lacking only AT&T's Band 30. That model will eventually have slower performance on AT&T's network than the A1633 model, although it'll still work generally fine.
AT&T will sell the A1633/34 model. You'll be able to roam with that and switch it to T-Mobile if you want, but you wouldn't be able to switch to Sprint or Verizon. If you want a phone that could work on all four carriers, make sure to get an A1687/1688.