Now that I've ruled out unique creatures, I'm gonna also rule out elementals. They tend to be presented very simply and are thus boring. While demons (and devils and daemons and whatever nonsense category they're in this week) are my favorite category by far, because I like the bad guys, my favorite outsider is actually a good one. Lawful good, in fact. My favorite outsider is the Hound Archon. First off, they can turn into dogs. That immediately makes them awesome surprise guests. Remember that dog that you've seen around town? Surprise, kids, he turns into a Hound Archon and smacks you down for breaking into that potion shop last week! One of my friend's games used a Hound Archon in dog form following us around as a guardian from the gods. It was sweet when the big reveal came.
Wanna play fetch with your head?
On top of that, Hound Archons are pretty beastmode combatants. For a low-mid level monster, they hit pretty hard, have a decent armor class, are super mobile (at will greater teleport), and with darkvision, low-light vision, scent, and at will detect evil, and an awesome perception bonus, it's hard to get away from them. Their spell like abilities add some nice utility as well, and, in general, they're a really good option for a fight or ally. They don't outshine the party usually as an NPC who helps the party along. In short, they're exactly what a holy enforcer should be. Plus, they're dogs. They're adorable.