Where we watch depends on what parade it is. For MSEP, we love Frontierland, across from the Pin Trading outpost. It's fairly dark, so photos won't come out well unless you have a camera designed for very low light, but it's less crowded, you don't have to camp out in your spot as early as on Main Street, and you can run back to BTMRR after the last float passes and ride 1-2 times before the wait builds up again.
For FOF, we like the same spot because it's shady, and again less crowded. That said, the old FP+ spot in the hub is amazing for photos with the castle behind the floats. But it's in full sun and VERY HOT. I'd also love to catch it from the upper level of the train station someday, because of the height of the floats, but I'm never willing to give up park time to squat in those spots.
For the special parades (Boo to You and
MVMCP), we love town square, in front of the theater where Mickey and Tink meet. We usually wait for the 2nd parade, too, and find our spots after fireworks and picking up a treat somewhere. Gives us easy/quick access to the exit, which is always welcome after a long party day.