I think the best viewing area is in front of Germany. But anywhere in World Showcase where your view won't be blocked by trees or other obstructions is good.
If you're eating inside at Rose and Crown, they'll let you in a reserved outdoor viewing area a few minutes before Illuminations.
Depending on the season, you might need to find a viewing spot an hour or more before the show, IF you don't want to be looking over anyone's shoulder, or IF you want to sit on one of the few available benches. If you're going in the fall, there's a good chance you can wait until a few minutes before it starts to find a spot, and still have no one standing in front of you.
A lot of the advice you see about getting to shows hours early, or arriving before park opening if you hope to ride many rides, is mostly for peak-season guests (middle of summer, holidays like Easter, Christmas and Thanksgiving).