So, I looked through the MM to see if I could find some good Ranger Companions, and I found some pretty crazy stuff:
First off, Giant Owls possess their own language. I repeat, Giant Owls possess their own language. Now I have to see if I can get the whole party to pick up Giant Owl as a language so that we have some secret communication.
Secondly, Elk, Giant Poisonous Snakes, and Panthers are pretty clear winners - Giant Constrictor Snakes especially, since their attack becomes:
+[6 + Proficiency] to hit, reach 10 ft., one target. Hit: 1d4 + [4 + Proficiency] piercing, and then the target has to make a DC 11 Strength save or take 3d6 + Proficiency poison damage on a failed save, or half of that on a successful one.
Ranger Companions make pretty excellent mounts if you read Ranger Companion as trumping the normal action limitations on unintelligent mounts. Because if they do, anything with Charge (Hi Mr. Elk!) is a winner. If it doesn't, stuff like Giant Wolf Spiders or Giant Lizards can net you a mount that can carry you all-terrain inside a dungeon, replaced with stuff like Giant Owls if your campaign is mostly out in the open. Did I mention Giant Owls don't provoke opportunity attacks for moving away from enemies?
Giant Owls would also probably fall under the "intelligent mounts" heading, given that they have Int 8.
Oh, and as a side note: you can get Swarms of Bats, Rats, or Ravens as a Ranger Companion.