To avoid repeating some popular choices that have already popped up on this thread...
I'd say Loyalty should be considered a classic. It doesn't really have any aspects that make it stand out above your average brickfilm fare, but then again, neither does Nathan Wells' Beast. It was one of the best brickfilms to come out of BRAWL 2014 (in my opinion), and, I still watch it every once in a while when the inkling calls.
Another brickfilm I revisit often is Doug Vandegrift's Pirates!. While not as revered as America: Outlawed, I think this brickfilm is equally as enjoyable. Some of the water effects still hold up really well, and the score is fantastic!
This may be a bit too early to call, but, I'd like to think that Squash's About will be another often cited and watched brickfilm. There's really not much that can be said of it, since, it really speaks for itself. I'd recommend you all go watch that if you haven't already/don't remember it.