As soon as I watched 'Season 7, Time for a Wedding' I fell in love with Garth! He actually makes the episode somewhat bearable to watch (like, it's really weird and uncomfortable).
He's an incredibly charming, happy and chilled out dude! Every other hunter you meet in that show is miserable and brooding (Dean...), which makes Garth's personality shine just that much more. He's like the fucking sun! He takes his craft seriously but not too seriously if you get me.
I love that he's sort of bad at being a hunter at the start; the first two episodes you he's in have the gag that he gets knocked out by whatever they're fighting and misses the final showdown and it's really entertaining.

I also love that he gets an actually decent ending. Supernatural is notorious for giving their characters (especially fan favourites) really shitty endings to their stories (most of the time they just get killed off). Garth manages to dodge all of that, and ends up semi-retired with a wife, kids and a dental practice. It's all very sweet and I can't imagine any other character slotting into this role as well as he did.
And even though he's retired, he's still willing to help Sam and Dean out. He's incredibly reliable, sort of like a rock. A big, cuddly rock. He's so stinking cute!