Don’t get me wrong, the Alvin Maker series by Orson Scott Card is amazing too, but for me, the sheer scale and complexity of Turtledove’s In the Balance series is awe inspiring. Alien lizard people invade Earth during World War II. The aliens are far more advanced technologically, but humans are a brutal low-tech monster race to the aliens.
The cast of characters is more like A Song of Ice and Fire than your standard SF series, but the wide view allows us to see how an entire world (ours specifically) reacts to the aliens, and how the aliens react to us. His world building is rich, deep, and nuanced. The characters react in realistic ways to situations they never expected to find themselves in. There are very human moments, very alien moments, and a few scenes that blur the line between the two.
If you’ve never read the series, I whole-heartedly encourage you to pick it up (preferably at an independent bookstore or check it out from your local library, but whatever you do read it!). I aspire to improve my craft to tell such a wide view of events, as compellingly as Mr. Turtledove. <Emphatic cough>