Outsider

Outsiders are freaking awesome.  Of course, they also have the largest range of types, because they can be just about anything.  From demons to angels to fire spirits to horrors from beyond the stars, outsiders have a giant depth of possibilities.  The question for today also lumped immortals into the mixture, which further convolutes my choices, because I do love some Dorian Grey style immortality, but I'm going to stay away from those.  It feels like cheating picking something that is a unique creature.  That's also why I'm going to eschew picking my favorite Demon lord, Demogorgon, although he's awesome and a half.  I mean, look at the picture below.  You'd expect that to be a feral beast.  He has an intelligence that makes most high level spellcasters look like Forrest Gump.  Love. It.

So cool.  

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.