Valley Panthers - A little more powerful than everything else, but not by much. Rewarding, and satisfying to become powerful enough to beat. Generally average stats, when you vanquish it you can probably either hit it, or brick it, but not both, and almost exactly when it's a high kill, you can usually do both. Great feeling of progression.
Valley Cougars - There is something subtle about the stats that I don't want to spoil which makes these really interesting to fight. Back when they were created, they were possibly the only monster to use this trick well. Ground breaking.
Cave Cobras - Not ground breaking, but terrifying. I love this type of creature.
Stinging Beetles - The feraler that demands your attention, and teamwork, to take down. Very fun (sometimes aggravating).
Sasquatch - Boring, plentiful, rewarding creature. Too slow for my liking, and the stats aren't interesting to me, but always good to have some staple for an area.
Angry Mother Sasquatch - Boss monsters are the best.
On top of that, the creatures are all pretty close in level, but far enough that you get a great sense of progression as you get stronger (see Valley Panthers). I think the area was ahead of its time, and you can usually identify these creature archetypes (and the way they're spawned) in all the areas I've created. When I'm stumped and can't think of good stats for a creature, I still just take the valley panther and scale it up. I'm always happy with the result. And I've taken the idea of annoying feralers well beyond where it should ever go.
I think I've done my fair share of innovation, but the Valley is still a big source of inspiration. You really can't go wrong copying that formula.