I vote sophomore year. My daughter spent her junior year in Australia through CIEE and loved it, but there were some issues with college applications. For example, her friends in the USA took a lot of AP courses junior year, which weren’t offered in AU. So her college applications showed only 4 AP classes, because that’s all she could fit in during senior year.
Also, she took the ACT in the spring before she left - at the end of sophomore year. She took it in the spring the next year after studying in AU. She got the exact same score. Schools abroad don’t “teach to the test” like they do in the USA. Had she stayed in the USA for junior year, and was taught more of the subjects typically on the ACT, I’m sure her ACT score would have gone up.
She also missed the chance to join the National Honor Society, and to take the tests required to become a National Merit Scholar. I imagine that securing leadership positions in student clubs would be harder too (would you vote for tennis captain a guy who missed a whole season?)
Finally, the Australian school calendar follows the calendar year. So when she arrived (in July, actually) it was the middle of the school year. After Christmas break, they began a new year.
To sum it up - don’t go abroad for the academics. Go for everything else. She loved it, would do it again (and is off to college in a different country) but junior year is hard. Do-able, but hard. I recommend sophomore year (or maybe one semester junior year). Good luck!